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A collection of comments and articles on the many aspects of bullet casting by various cast bullet shooters
Cast Bullets For Beginner And Expert
SECOND EDITION, 2007 - Joe Brennan
Chapter 2.2 Bullet Design And Fit

     How To Scale Bullets Up Or Down

     Sometimes we wish to scale a bullet design up or down, to design-for example-a 25 caliber bullet that is a "scale model" of a certain 30 caliber bullet.

     The Ballistic Coefficient (BC) of a bullet is a measure of how efficiently that bullet goes through the air; of how little velocity is lost as the bullet travels down range.

     If two bullets are fired at the same muzzle velocity, and if one has a higher retained velocity at (say) 200 yards, then the bullet with the higher retained velocity has a higher BC.

     Bullets with the same BC, fired at the same velocity, follow the same path from muzzle to target. So, for instance, if we fire 22 caliber and 45 caliber bullets with the same BC and muzzle velocity, in the same conditions; then the trajectories of the bullets will be the same, and they will both be affected by the wind to the same extent. The higher the BC, the less the bullet is displaced by the wind and the less it drops at any range, for a given muzzle velocity.

     So if -back to our example-we want a 25 caliber bullet that is a "scale model" of a certain 30 caliber bullet, what we really want is a 25 caliber bullet that has the same BC as that certain 30 caliber bullet.

     There are two ways to design this new bullet, the easy way and the hard way. (Keep in mind the fact that we're using approximations, and that the new bullet will have APPROXIMATELY the same BC as the original bullet.)

     Dimensions of the new bullet should be determined by the dimensions of the barrel that the bullet is to be shot in.

The Easy Way

     Draw the original bullet, with dimensions. Lets say the 30-caliber bullet is a bore-rider, with nose of .302", base bands of .311", and length of 1.1".

     Then copy the drawing and change the diameters to those desired. Then the 25-caliber bullet drawing might have a nose of .251" and base bands of .259".

     Make the new bullet as long as the original bullet. The 25-caliber bullet would then have a length of 1.1", the same as the 30-caliber bullet. Elongate the new bullet design by making either the base bands or the nose or some combination longer.

     N.B. We're designing a bullet with the same BC as the original. In our example, we've ended up with a 25-caliber bullet 1.1" long. This may or may not stabilize in a particular rifle, depending on the twist. For a 25-caliber rifle, the Greenhill formula tells us that a twist of one turn in nine inches, or faster, will stabilize a bullet 1.1" long, and the 25-caliber bullet will weigh 144.5 grains. (calculations below)

     This "Easy Way" is based on the notion that bullets of the same design and length have approximately the same BC. A 1" long 45-caliber bullet has the same BC as a 1" long 30 caliber bullet-approximately.

The Hard Way

  • 1. Draw the original bullet, with dimensions. Lets say the 30-caliber bullet is a bore-rider, with nose of .302", base bands of .311", and length of 1.1".

  • 2. Re-dimension the drawing proportionately. The 25-caliber bullet would then have a nose of .251", base bands of .259", and a length of .909". The .909" is the scale model length-the ratio of bullet diameters is .257/. 311, multiply this by the 30-caliber length of 1.1", and get .909".

You now have a 25-caliber bullet design that is a "homologue" of the 30-caliber bullet design.

BUT, while the 25-caliber bullet is a model of the 30-caliber bullet, it does NOT have the same BC.

  • 3. Lengthen the 25-caliber bullet such that it has a Sectional Density equal to the 30-caliber bullet. What we're doing here is figuring out how much the new bullet must be lengthened to make the SD and BC equal the 30 caliber bullet - how long the new bullet must be. How to do that is explained in excruciating detail below.

     Ballistic Coefficient is related to Sectional Density (SD). Bullets with the same form or shape and the same SD have the same BC.

     SD = Weight/Diameter Squared, where the weight is in pounds and diameter is in inches. Then SD is measured in pounds per square inch.

     The 30-caliber bullet weighs 208 grains in wheel weights.

     The SD of the 30-caliber bullet is calculated thus:

     Weight in pounds is 208 grains/7000 grains per pound = .029714 pounds.

     Diameter squared is .311" squared = .096721 square inches

     Weight/Diameter Squared= SD = .029714/. 096271 = .307216 pounds per square inch.

     How much does the 25-caliber homologue weigh?

     A .308 diameter cylinder 1" long, made of wheel weights, weighs 210 grains. (I use the .308" cylinder dimension and weight for convenience, as an approximation of the bullet diameter average. Remember-this is an approximation.)

     Then a .308" diameter cylinder 1.1" long, made of wheel weights, weighs 1.1 X 210 = 231 grains.

     The 30-caliber bullet weighs 208/231 = 90% as much as a wheel weight cylinder as long as the bullet.

     It seems reasonable to assume that the same ratio, 90%, holds for the 25-caliber bullet.

     A .257" diameter, 1" long wheel weight cylinder weighs 146 grains. Then a .909" long .257 diameter cylinder weighs .909 X 146 = 132.7 grains, and the bullet would weigh 90% of that, or 119.4 grains.

     How much should the 25-caliber bullet weigh, for the SD's of each to be equal?

SD = Weight/Diameter Squared

SD of 30 caliber bullet  = .307216

Diameter of 25 caliber bullet = .257"

Diameter squared of 25 caliber bullet = .066049" sqrd.

SD = .307216 = Weight of 25 cal bullet/. 066049" sqrd.

Weight of 25 cal bullet = .307216 X .066409 = .020291 pounds, X 7000 = 142 grains

     How much weight must be added to the homologue bullet?

     Should weigh = 142, minus Does weigh, 119.4 = 22.6 grains must be added.

     How much length must be added to the homologue?

     A .257" diameter, 1" long wheel weight cylinder weighs 146 grains.

     A 22.6 grain .257" diameter cylinder is 22.6/146 = .155" long.

     Then the homologue 25-caliber bullet .909" long must be lengthened .155" to a total length of 1.064" for the 25-caliber bullet to have the same SD and BC as the 30-caliber bullet.

     Compare The Easy Way and The Hard Way

     Easy way-length = 1.1", weight = 144.5 grains.

     Hard way-length = 1.064, weight = 146 grains.

     Here's a table of weights of lead alloy cylinders 1" long, various diameters and alloys. Do I have a lot of time on my hands?

Lead/Alloy > >

 

Lead

5 sn

10sn

Wheel

Linotype

Lyman

Monotype

95 pb

90pb

weights

#2

Specific gravity > >

 

11.34

11.00

10.50

11.14

10.63

10.63

10.10

 

Cylinder

 

 

 

 

 

 

 

 

Volume

Weight

Weight

Weight

Weight

Weight

Weight

Weight

Caliber

cubic "

Grains

Grains

Grains

Grains

Grains

Grains

Grains

0.224

0.039

113

110

105

111

106

106

101

0.244

0.047

134

130

124

132

126

126

120

0.257

0.052

149

144

138

146

140

140

133

0.264

0.055

157

152

146

154

147

147

140

0.277

0.060

173

168

160

170

162

162

154

0.284

0.063

182

176

168

179

170

170

162

0.308

0.075

214

208

198

210

201

201

191

0.311

0.076

218

212

202

214

204

204

194

0.323

0.082

235

228

218

231

221

221

210

0.338

0.090

258

250

239

253

241

241

229

0.357

0.100

287

279

266

282

269

269

256

0.375

0.110

317

308

294

312

297

297

282

0.410

0.132

379

368

351

372

355

355

338

0.429

0.145

415

403

384

408

389

389

370

0.457

0.164

471

457

436

463

441

441

419

     Note that if the new bullet is SMALLER in diameter than the original bullet, then we must ADD a cylindrical section of the new bullet to make sectional densities and BC's equal; and that if the new bullet is BIGGER than the original bullet, then we must REMOVE a cylindrical section of the new bullet to make the sectional densities and BC's equal.

     Note that the EFFECT of a change in BC is not proportional to that change. The BC determines, for any given muzzle velocity, how much the bullet will drop, and how much the bullet will be affected by the wind, at any range. For example:

     Muzzle Velocity = 1500 fps// G1 ballistic table//Deflection from 10 mph wind.

BC .400 .350
200 Yd Drop 35.5" 36.2"
200 Yd. Deflection 7.5" 8.6"
1000 Yd. Drop 1371.8" 1449.0"
1000 Yd. Deflection 164.4" 182.5"

     A decrease of BC from .400 to .350 will increase the drop and wind deflection of the bullet. BUT, not proportionately; since reducing BC from .400 to .350 is a reduction of 12.5%, and: 200 Yd Drop increases 2%.

 

  • 200 Yd. Deflection increases 15%

  • 1000 Yd. Drop increases 6%

  • 1000 Yd. Deflection increases 11%.

     The point is that small changes in the BC yield some smaller changes in bullet behavior, and time spent on assuring that the newly designed bullet has the identical BC as the original bullet is better spent in other pursuits-perhaps developing a bullet lube recipe containing yak butter.

     Note also that some of the above work is based on an article written by me for the ASSRA News in the distant past, and that I learned most of the hard stuff (that homologue business) from articles written by others in The Fouling Shot including W. C. Davis. The only things original with me are the observation that BC is approximately constant, for similar-shaped bullets of the same length, regardless of caliber; and that fascinating table of weights of cylinders of different alloys.

Ballistic Coefficients (BC)

     The BC of a bullet is a measure of how efficiently that bullet goes through the air; of how much velocity is lost as the bullet travels down range.

     BC's are measured in decimals such as, 269 or .361. The higher the number, the higher the BC, the less the effect of wind on the bullet and the higher the retained velocity of the bullet.

     The BC of a bullet varies with muzzle velocity, air density, wobble or yaw, and the time of high tide. I believe that three digits to the right of the decimal is one too many, and that BC should more properly and accurately be recorded as .27 or .36. The path of the bullet does not vary proportionately with variations in BC. If two bullets are fired at the same muzzle velocity, and if one has a higher retained velocity at (say) 200 yards, then the bullet with the higher retained velocity has a higher BC.

     Bullets with the same BC, fired at the same muzzle velocity, follow the same path from muzzle to target. So, for instance, if we fire 22 caliber and 45 caliber bullets with the same BC and muzzle velocity, in the same conditions; then the trajectories of the bullets will be the same, and they will both be affected by the wind to the same extent. The higher the BC, the less the bullet is displaced by the wind, at any given muzzle velocity.

     The BC of a bullet is a function of the form (shape) of the bullet, and of its sectional density (SD). Sectional Density is Weight/Caliber Squared, units are pounds per square inch. Bullets with the same form and the same SD have the same BC.

     The effect of form on BC can be thought of as having three parts. The form component is determined by the form of the nose, the form of the base, and the form and length of the side. The form of the nose and the base are very important determinants of BC, but the form and length of the side is not an important determinant of BC.

About wind drift and velocity.

     BC, bullet velocity and wind speed determine how much a bullet is deflected by the wind on the way to the target.

     BC and bullet velocity don't matter when you're shooting in the calm, with no wind. I prefer shooting when there's no wind, although the wind, especially a gusting wind, is a wonderful excuse.

     Here's a table showing the wind deflection (wind drift) of bullets with BCs varying from .2 to .6, and velocities from 800 fps to 2800 fps. Range is 200 yards, the wind is coming straight across the range, 90 degrees from the line of flight of the bullet.

     This table was constructed using a ballistic program named "Balistic" by a fellow named "Frenchu". I've had the program for years, and used it to estimate long range sight settings before shooting at those long ranges, and its never let me down.

Velocity, Deflection and BC 200 yard deflection (in inches) in 10 mph 90 degree wind
fps BC = .2 BC = .25 BC = .3 BC = .35 BC = .4 BC = .45 BC = .5 BC = .55 BC = .6
800 9.1 7.2 6.0 5.1

4.5

4.0 3.6

3.3

3.0  
900 9 7.2 6.1 5.2 4.6 4.1 3.7 3.3 3.1
1000 9.8 8 6.7 5.8 5.1 4.6 4.1 3.8 3.5
1100 11.4 9.4 7.9 6.9 6.1 5.5 5.1 4.6 4.2
1200 13.1 10.8 9.2 8.0 7.1 6.4 5.8 5.3 4.9
1300 14.4 11.8 10.0 8.7 7.7 6.9 6.2 5.6 5.2
1400 14.9 12.2 10.2 8.8 7.7 6.8 6.1 5.6 5.1
1500 15 12.1 10.1 8.6 7.5 6.6 5.9 5.4 4.9
1600 14.8 11.8 9.7 8.3 7.2 6.4 5.7 5.2 4.7
1700 14.2 11.2 9.2 7.8 6.8 6.0 5.3 4.8 4.4
1800 13.5 10.6 8.6 7.3 6.3 5.6 5.0 4.5 4.1
1900 12.8 9.9 8.1 6.8 5.9 5.2 4.6 4.2 3.6
2000 12 9.3 7.6 6.4 5.5 4.9 4.4 3.9 3.6
2100 11.3 8.7 7.1 6.0 5.2 4.6 4.1 3.7 3.4
2200 10.6 8.2 6.7 5.6 4.8 4.3 3.8 3.4 3.1
2300 10 7.7 6.2 5.2 4.5 4.0 3.6 3.2 2.9
2400 9.3 7.2 5.8 4.9 4.2 3.7 3.3 3.0 2.7
2500 8.8 6.8 5.5 4.6 4.0 3.5 3.1 2.8 2.6
2600 8.3 6.4 5.2 4.4 3.8 3.3 3.0 2.7 2.4
2700 7.8 6 4.9 4.1 3.6 3.1 2.8 2.5 2.3
2800 7.4 5.7 4.7 3.9 3.4 3.0 2.7 2.4 2.2
 

Here's the same data as a graph.

Here's a table showing the wind deflection (wind drift) of bullets with BCs varying from .2 to .6, and velocities from 800 fps to 2800 fps. Range is 1000 yards, the wind is coming straight across the range, 90 degrees from the line of flight of the bullet.

This table was constructed using the ballistic program named "Balistic" by "Frenchu".

1000 yard deflection (in inches) in 10 mph 90 degree wind
velocity                
fps BC = .2 BC - .25 BC = .3 BC = .35 BC = .4 BC = .45 BC = .5 BC = .55 BC = .6
800 261.9 200.3 162.4 136.7 118.1 104.0 93.0 84.1 76.8  
900 244.3 188.8 154.5 131.0 113.9 100.8 90.5 82.1 75.2  
1000 241 189.1 156.6 134.1 117.6 104.9 94.8 86.6 79.7  
1100 248.4 198.2 166.4 144.3 127.8 115.0 104.8 96.3 89.2  
1200 260.1 210.7 179.1 156.9 140.2 127.1 116.5 107.7 100.2  
1300 271.4 222.4 190.7 168.2 151.1 137.5 126.5 117.2 109.4  
1400 280.4 231.6 199.6 176.6 159.0 144.9 133.4 123.6 115.3  
1500 287.4 238.4 206.0 182.5 164.4 149.8 137.7 127.4 118.6  
1600 292.5 243.4 210.5 186.5 176.7 152.6 139.9 129.1 119.8  
1700 295.6 246.2 212.8 188.2 168.8 153.1 139.8 128.6 118.7  
1800 297.2 247.4 213.5 188.2 168.3 151.9 138.2 126.4 116.2  
1900 297.6 247.4 213.0 187.1 166.5 149.6 135.3 123.1 112.6  
2000 297 246.4 211.4 184.9 163.7 146.3 131.6 119.1 108.3  
2100 295.5 244.5 208.9 181.7 160.1 142.2 127.1 114.4 103.5  
2200 293.2 241.7 205.5 177.8 155.6 137.3 122.0 109.2 98.4  
2300 290.3 238.3 210.5 173.3 150.6 132.0 116.6 103.8 93.1  
2400 286.9 234.4 197.1 168.3 145.3 126.4 111.0 98.4 88.0  
2500 283.1 230.1 192.2 163.0 139.7 120.8 105.5 93.1 83.2  
2600 278.9 225.5 187.1 157.5 133.9 115.0 100.0 88.1 76.5  
2700 275.4 220.6 181.7 151.8 128.1 109.4 94.8 83.4 74.2  
2800 269.8 215.5 176.2 146.0 122.3 104.0 89.9 78.9 70.3  

 

Here's the same data as a graph:

 

Here's a table showing the bullet drop of bullets with BCs varying from .2 to .6, and velocities from 800 fps to 2800 fps. Range is 1000 yards.
This table was constructed using a ballistic program named "Balistic" by a fellow named "Frenchu".
1000 yard bullet drop in inches
Velocity                
fps BC = .2 BC = .25 BC = .3 BC = .35 BC = .4 BC = .45 BC = .5 BC = .55 BC = .6
800 4335 3924 3680 3519 3404 3318 3251 3198 3154
900 3509 3175 2975 2841 2748 2672 2616 2571 2533
1000 2979 2689 2513 2395 2309 2243 2192 2150 2116
1100 2638 2373 2210 2099 2018 1956 1906 1866 1932
1200 2400 2151 1996 1889 1810 1749 1700 1659 1625
1300 2215 1976 1826 1721 1643 1582 1533 1493 1458
1400 2056 1826 1679 1576 1499 1438 1388 1347 1312
1500 1918 1694 1551 1449 1372 1311 1261 1219 1183
1600 1796 1577 1436 1335 1258 1197 1147 1104 1069
1700 1683 1470 1331 1231 1154 1093 1046 1000 964
1800 1589 1372 1235 1135 1059 998 947 905 870
1900 1486 1281 1146 1048 972 911 861 820 785
2000 1399 1198 1065 968 893 833 784 743 710
2100 1318 1121 990 894 820 761 713 674 642
2200 1243 1049 920 825 752 695 649 612 582
2300 1172 982 855 762 690 635 591 556 529
2400 1107 920 795 703 634 580 539 507 482
2500 1045 862 739 649 582 531 493 463 441
2600 987 808 687 600 535 487 451 425 404
2700 933 757 639 554 492 447 414 390 372
2800 883 710 594 512 453 411 382 360 343

 I just have to study these tables and graphs for a while before I see what's going on.

Here's the same data as a graph:

     Bullets have LESS wind drift at 1100 fps than at higher velocities. As velocity is increased, wind drift increases. Then as velocity goes above 1300-1500 F/s, wind drift again decreases. This is counter-intuitive, but it is true. Unfortunately, wind drift is highest at 1300 fps to 1500 fps, the middle of the low speed cast bullet velocities.

     We can reduce wind drift by either increasing or decreasing muzzle velocity or by increasing the BC of the bullet. The most feasible options seem to be either reducing muzzle velocity or increasing the BC of the bullet or both.

     The easiest and cheapest option is to reduce the velocity of the bullet by reducing the powder charge. Reducing the powder charge sometimes/often increases variation in MV because the powder takes up less of the space in the case, and reduces accuracy.

     One way to reduce MV variation in light charges is to use a wad to hold the powder in place against the primer. See "ON WADS AND FILLERS" for a discussion of the possible risks of using wads.

     Bullet drop, with BC above ~.3, isn't sensitive to BC. A rule of thumb that I've developed is: Bullets of the same length and same shape have about the same BC, regardless of caliber". A 1" long 22-caliber bullet has about the same BC as a 1" long 30-caliber bullet.

     Another rule I've developed is: "Don't worry about BC". It's nice to know about, and some of the more/most advanced cast bullet shooters take it into consideration, but for the majority of us it shouldn't be a concern.

Thoughts on Throats, Leade, Ball Seats and Bullet fit

Ric Bowman

     A lead alloy bullet has a hard life. It is not strong nor does it have a hard copper jacket. It gets mushed by a heavy gas check or a wall of high temperature gas in the butt. It has a hard steel wall that confines it and a column of air holding it back. Then the lands grab it and make it twist around its center of form. No, an alloy bullet doesn’t have an easy life.

     The ideal position for the bullet is this: the nose is snugly resting on the top of the lands and the front driving band is snug against the leade with its circumference is exactly the same as the diameter of the throat. The rest of the bullet is contained in the neck of the case that is perfectly centered in the perfectly centered chamber. The case mouth touches the front of the chamber so that a smooth surface exists from the base of the bullet through the throat, leade and bore. There is no gap between the front of the case mouth and the throat, there is no gravity, ejector or extractor tension to keep the resized case from laying perfectly concentric to the bore center line. Yeah, when pigs fly and I’m a young man again!

     Terms as I use them, whether they are technically correct or not:

  • Chamber - The interior portion of the barrel that surrounds the brass case.

  • Throat – The parallel section of the bore in front of the chamber before the lands of the rifling begins.

  • Leade – The section of the bore in which the lands of the rifling rise up from the throat into the rifled bore proper.

  • Ball seat – A type of throat/leade combination that is made with a one- or two-angle cut from the front of the chamber to the bore proper.

     Illustration #1 is the profile of a good cast bullet chamber; it is not to scale but shows the principles required. The fired case neck has expanded to just release the bullet with no excess room for the bullet to move. The front of the neck approaches the ledge cut into the barrel for the neck. The throat is long, parallel, and only slightly larger than the groove diameter. The leade is long and of shallow angle to allow the bullet body to smoothly enter the bore. This chamber would shoot long heavy-for-caliber bullets well.

     Illustration #2 is of two common types of ball seat chambers; again not to scale. “A.” is the best possible one, with the fired case neck just slightly larger than the groove diameter. “B.” is most commonly encountered with the fired case neck diameter much larger than the groove diameter. The major difficulty with this design is that the soft cast bullet is not contained by either the case neck or the chamber. This leads to uncontrolled obturation, bullet tipping, gas cutting and driving band shaving. While it works OK with jacketed bullets, you must use a very hard alloy to have consistent performance. With anything softer than linotype cast bullets accuracy is usually poor.

     There have been many attempts to design bullets that minimize the reality of factory-produced rifles. Also there are reloading techniques that minimize bullet/ barrel interface errors.

     Harry Pope designed a bullet for fixed cartridges that had a nose that rode on top of the lands, two driving bands of increasing size to fill most throats and a base band that is a tight slip fit into a fired case neck. In theory, the bullet and case are centered. With alloy and powder charge / pressure curve matched, the bullet obturates to completely fill the barrel and provides consistency. It works but it has low velocity potential, requires much experimenting, and load changes for varying temperatures and bore conditions to shoot well. (This bullet is the Pope Lyman 308403 or 311403, which I have found to be very easy to load accurately with small charges of Unique..)

     In the early 1900’s, Ideal produced several designs for their new invention, gas checks, which are still working well. The classic is 311284 (308284) a gas check design for the 30 US Army. About 40% of the bullet is a bore-riding nose with two wide driving bands and a short gas check shank. The theory is that the long nose centers the front of the bullet with the bore, and if the bullet is sized correctly, the first band centers and fills the throat. Because the bullet is centered in the bore, it holds the resized case centered in the chamber.

     Bob Cramer (who sold out to SAECO) designed the “bore riding” bullet with 60% or more, of the bullet inside the bore, sitting on top of the lands. It works well with short case necks, ball seat chambers or when you want shallow seating. The bullet picks up, centers the partial resized case, and places the front driving band against the lands of the barrel.

     The bullet designs by H. Guy Loverin take a different tack. They have almost no nose with multiple narrow driving bands. The bullet is seated into a tight case neck and adjusted for a critical seating depth. Properly done, the first driving band is forced into the leade engraving the lands into the bullet. The driving bands must be close to throat diameter. The multiple lube grooves act as a pneumatic piston that uses gas turbulence over these grooves to help seal the throat. This design can be very effective as long as combustion gas pressure is not too high or alloy strength too low, leading to lube groove collapse. Many feel that this is the best bullet design for 2000+ f/s reloads with good accuracy.

Swaging bullets to fit the throat has become fairly popular in the last 25 years. Usually it works like this. A reamer of known dimensions is used to cut the throat to a known configuration. The same reamer is also used to cut a swage of the same dimensions. The bullet of choice is forced into the swage to shape it to the exact contour as the barrel throat. The bullet is seated long into a loose case neck. As the cartridge is chambered, the bullet stops in the barrel and the neck of the case slides up the sides of the bullet. This gives exceptionally consistent bullet placement in the barrel.

     Joe Gifford has developed a method called “float seating” that was made for the long throats of military rifles using wheel weights for alloy. While it is certainly effective there are some requirements: smooth bore surface, heavy-for-caliber bullet, an as cast diameter bullet nose that is a snug fit on top of the lands and a fast powder. This is how he does it for an 1891 Argentine Mauser with Lyman 311284. The case is sized only down pass the area of the neck where the gas check will end up, and then the neck is opened with a 0.311 Lyman “M” die. The bullet is sized in a 0.314 die, which only crimps the gas check onto the shank and leaves the bullet un-sized. The bullet is seated so that the entire nose is in the bore and the front driving band is hard against the leade. This leaves the top edge of the gas check touching the inside of the case neck, but none of the bullet does. This allows the bullet to self-center into the bore. He uses 12.0 grains of Unique for a fast pressure rise to insure full obturation of the body into the throat. In theory, this gives excellent bullet alignment in the bore and the hard fit insures good powder ignition. The disadvantages are that rounds are too long to feed from the magazine, the bullet is not secure in the case and a loaded round cannot be extracted without leaving the bullet in the barrel. However, with this technique you can stack your shots one on top of the other for fine group shooting.

     The type and dimensions of the throating also determines what type of alloy you need to use for your bullets. The closer the bullet fits the throat, the softer (read cheaper) alloy you can use within reason. Ball seats do not support the bullet well and require harder alloys compared to a snug parallel throat. Magazine length may limit how long you can seat the bullet. You cannot always seat the bullet long to fill the throat and leade to help control gas cutting. In addition, you cannot always depend upon obturation to seal the bore. Bullet choice should match the throat and seal as well as possible to prevent gas cutting. If the bullet doesn’t fit the throat well, you can try a slower burning powder, like IMR 4198 and slower, to delay peak pressure until the bullet is partially in the bore.

     Without slugging your throat, you are doomed to trying different moulds and loading techniques until you hit upon something by luck. A carefully made slug and use of a good micrometer will let you eliminate moulds that just do not cast bullets the size you need.

     This is a picture of a drawing made to SAMMI specifications for the 30/30 chamber. If you look where the end of the case would be, you will see a steeply sloping cut of 15 degrees all the way into the bore. This is what I call a ball seat.

     This is a picture of a drawing made to SAMMI specifications for the 30/06 chamber. If you look where the end of the case would be, you will see a steeply sloping cut of 35 degrees and 43 minutes. This only reduces the throat to .3106 inches, just over bullet size. Then there is the throat, reducing down at 1 degree and 22 minutes to bore size. This is what I call a throated chamber.

Fitting a Cast Bullet to the Chamber of a Firearm

Bill McGraw, Ric Bowman, Ken Mollohan

     Recent RCBS, Lee, SAECO and Lyman moulds cast bullets that fit many/most guns well; Lyman moulds of older manufacture usually cast much larger than newer molds. There are other moulds made by NEI and many other custom mould makers, and there are many used moulds for sale at gun shows and other outlets. In most cases you can buy a mold that casts a bullet that shoots well in your gun.

     Read a good loading manual before loading any ammunition. Follow directions, double check powder type and charges, and look into the cases to insure the powder level is correct before seating the bullet.

     Revolver bullets must fit the cylinder throats, must be throat diameter (exact throat diameter or -.0005" inches minimum). If the bullet is much smaller than the cylinder throat, then there will be gas blow-by, leading, and accuracy will be destroyed.

     If the forcing cone is slightly larger than the cylinder throats and tapers to the groove diameter, all is well.

     If the barrel groove diameter is smaller than the cylinder throats, all is well.

     But, if the groove diameter is larger than the cylinder throat diameter, problems will ensue. The only option is to have the cylinder throats reamed to bore diameter.

Rifle bullets must fit the throat, must be no more than half-a-thousandth of an inch smaller than the throat. The throat ( or "leade" or "ball seat") is the section of the chamber from the end of the case to the rifling. The point is that the bullet must make a seal to keep the gas from blowing by and depositing lead on the bore.

     Fitting cast bullets is about dimensions of chambers, throats and of bullets and bullet moulds. If the bullet is "too small", upon firing the gas will blow by the bullet, gas cutting the bullet and leading the barrel. The intent is to fit the bullets so that gas-cutting and leading is minimized, so that the gun shoots accurately, and so that SAFETY is maintained.

These dimensions are needed to fit a bullet:

For Revolvers:

  • a. cylinder throat diameter

  • b. barrel bore and groove diameters at the muzzle and end where the barrel screws into the frame.

For rifles:

  • a. throat dimensions

  • b. barrel bore and groove diameters at the breech end and the muzzle end to see if there is any taper to the bore.

  • c. for rifles with a screwed-on or pressed-on sight or sling mount, bore and groove dimensions should be measured under the mount also

     Instructions for measuring these dimensions are included in this book.

Additional considerations:

  • 1. A larger bullet is better than a too-small bullet. The smaller bullet may not shoot without gas-cutting and eventual bore fouling.

  • 2. Lever action rifles with tube magazines require flat-nosed bullets that are crimped in place. Pointed bullets MAY fire the next cartridge in line by hitting the primer.

  • 3. Bolt action rifles may allow a tighter fit in the throat so as to allow the bullet to seat in the axis of the bore yet not so tight that the bullet will lodge or debullet if the loaded round needs to be extracted. Except sometimes bolt guns are more accurate when the bullet is seated way out, and the cartridge can't be removed from the gun without de-bulleting. Every shooter should and will pull the bullet out of the cartridge when removing a cartridge from the gun. The powder falls out of the case and immediately goes everywhere in the action. Now the gun has to be disassembled and cleaned, and the bullet has to be knocked out of the bore. This de-bulleting is especially interesting if it occurs during a hunt or a match.

  • 4. In all firearms, some leeway in fitting must be considered if many rounds are to be fired in match competition when fouling buildup is expected and cleaning is not done until such competition is finished.

  • 5. Loaded rounds should fit in a auto-load pistol and lever, slide or auto-load rifle’s magazine, if you load cartridges in the magazine. I never load rifle cartridges in the magazine, I shoot all rifles one cartridge at a time. At the Old Colony Sportsman's Association in Pembroke MA. this is the rule, single loading always unless under a special dispensation (M1s, M1As, AR15s etc.).

  • 6. Cartridges should chamber without having the bullet seated too deeply in the cartridge case. In the case of bottle-necked cartridge cases, the bullet base should be seated no deeper than the bottom of the neck if at all possible.

  • 7. For bore-riding bullets, the bullet base should be of groove diameter and the bullet nose of bore diameter plus 0.001"-0.003”.

  • 8. Lower velocity loads work well with the larger diameter bullets; higher velocity loads seem to work best with the smaller diameter increase .

  • 9. There is much difference in chamber and throat dimensions from one gun to another.

     There is one situation where the general rules do not work well. Some chamber necks are too small to seat a cast bullet of proper diameter for the neck, throat and bore and groove dimensions.

     One example is the Browning copy of the Winchester Lever Action rifle  in 348 Winchester. John Rhodes has reported on his rifle’s inability to shoot with CB’s. The chamber is made for jacketed bullets of much smaller diameters than needed for CB use.

     Other examples include the 38 and 41 Colt, and some transitional rifles such as the Springfield Trap Door, Werndl and Snider.

In these cases there are several alternatives to fix the problem.

  • 1. Open the throat with a throating reamer. (This should be the LAST choice.)

  • 2. Breech seat the bullet in front of the case in rifles.

  • 3. Turn case necks or make step-necked cases to allow larger bullets to be used.

  • 4. Use a "Heel" or "hollow based" smaller-diameter bullet. Below are examples of pistol bullets, the information applies to rifle bullets as well.

     The 359417 is a conventional design bullet. Sometimes this bullet style, loaded in the case, makes a cartridge that is too big in diameter for the chamber. The 386177 is a "heel" style bullet Where the "196 gr" is printed is smaller in diameter than further forward, allowing the bullet to be seated in the case and the cartridge to fit the chamber. The 387178 is a hollow based bullet that, when seated in the case, allows the cartridge to fit in the gun. Upon firing, the hollow base expands, sealing the bore and stopping blow-by and leading.

  • 5. Use a smaller bullet of soft alloy that makes a cartridge that fits in the chamber. With a fast smokeless powder or black powder the bullet may expand enough on firing to seal the bore.

  • 6. Military and worn barrel throats present a condition where bullet diameters are needed that are not available in standard moulds. Many factory sporter rifles have such large throats. In this case there are two ways to remedy this:

     Have a custom mould made or modify a current mould to fit such a throat, or

     Fit the bullet as recommended and use a wad under the bullet that will seal the throat when the round is fired. Such wads are made from paper, card and plastics and are cut with an arch punch. with a wad cutter using a drill press, or when a few wads are needed, cut with a cartridge case neck fitted and outside deburred to a knife edge so that cutting is consistent.

     Wads can be cut by using a mallet tapped on the cartridge cutter on the wad material backed by a soft backer of wood, leather, or a soft plastic so as not to bend the wad cutter; chucking the cartridge wad cutter in a drill press is a good option.

     If using the Lee Collet die to size cartridge necks for loading cast bullets, the same die can be used to adjust the cartridge case to cut different diameter wads until the diameter is suitable for the load and chamber throat. Such wads must be at least the diameter of the throat or slightly larger and should fit the cartridge neck under the bullet with no airspace between the wad and bullet and the wad must be seated in the case neck in such a way it cannot fall into the powder space of the cartridge.

     In no case should a loaded cartridge neck be larger than the chamber neck; the cartridge may not chamber and even if chambered, dangerous pressure could result. There should be enough clearance to allow the neck to expand on firing. Tight-chambered rifles built for and used by experienced cast bullet reloaders may have very little neck clearance. For the rest of us, the loaded cartridge neck should be a minimum of .002" smaller than the chamber. It is thought that greater neck clearance diminishes accuracy, so a maximum of .006" clearance should be the goal.

     Bullets can be sized down in size/lube dies to fit the groove dimensions. Nose diameters cannot be reduced without custom dies. A rather simple die is available from Don Eagan; such dies are made to reduce nose diameters and are also made to taper the entire bullet to fit a tapered throat. Don advertises in The Fouling Shot and can make a die to suit a particular throat. These dies are used in place of the regular size/lube dies in RCBS and Lyman lube/sizers; once all the bullets are modified, the die is removed and the regular die is replaced for seating gas checks and lubing the bullets.

An aside on restricted bores.

     Some revolver barrels are squeezed where the barrel is screwed into the frame.

     Some rifle barrels are squashed where a sight base or sling swivel base is swaged onto the barrel. It has been reported that some Ruger barrels with swaged-on sling swivel bases are squashed.

     When the barrel is squashed the bore and groove are squozen smaller, as much as .004"-'005" has been reported. This constriction in the barrel of rifle or revolver may/will substantially reduce accuracy, may be found and measured by slugging the barrel, and may be removed by lapping or fire-lapping the barrel.

"joe b, My Ruger #1 in 45-70 has a definite restriction where the bbl band sling swivel assembly has been installed. This can easily be felt with a tight patch on a cleaning rod. Why Ruger chose that method (swaging?) is beyond me. Possibly cheaper installation than either soft or hard soldering." Frank at Cast Boolits

     It has also been reported that these sight bases and swivel bases are soldered on, and scale is what causes the tightness in the barrel.

     "Any model Ruger that has the front sight band can have a constriction but doesn't necessarily have to. And even if it doesn't have a constriction they could have heated the steel so badly that it becomes scaled. I had that on a 77-44." John Robinson

     "You would think Ruger would break the code on how to solder ramps, bands and such. I have had to send three back for new barrels as you can't remove that heat scale stuff. The last was an NIB Old Army with scale where they soldered on the front sight and another small patch where they attached the loading lever stud to the barrel. They have always replaced the barrels with no fuss, but this must get costly for them."

Chargar at Cast Boolits

How to use the measured bore, groove and throat diameters to select a cast bullet that is likely to do well in your gun:

Ken Mollohan

     Generally speaking, cast bullets suffer from relative softness and low melting points. These form serious obstacles to obtaining good results from all but the lightest loads. The more protection you can provide from the gunpowder flames, the better your chances of good results. The tighter the fit of the bullet in the bore, the more support it will get, and the less likely it will be to collapse and shoot wildly.

     To this end, it is the usual practice to select an Ideal bullet design with a nose that is at least a tight ‘push fit’ in the bore. For example, a nose with a diameter of 0.301” to 0.305” generally works well in bores with lands that measure 0.300”, like the 30-06. Well worn barrels may even use nose diameters up to 0.312”. The deciding factor is whether the throat and leade are worn enough to allow the round to chamber with reasonable ease. Pistol bullets generally don’t have much of a bore riding nose, but can be considered much like Loverin bullets. Loverin bullet designs do not have a bore riding nose, and their diameter should be determined as for the throat, below. Revolver bullets should be sized to the largest diameter throat in the cylinder.

     It was once the practice to select a sizing diameter equal to the groove diameter of the bore, and this is still recommended in print today. This was the original inspiration for slugging a barrel in the first place: To determine the sizing diameter. However, sizing a bullet body to groove diameter means it will be substantially undersized as it lies in the neck and throat of the rifle. Undersized bullets are not well aligned with the bore, and they allow a lot of flame to jet around their sides, which leads to leading and other problems. Today, the practice of sizing bullet bodies to fit the throat of the chamber is well recognized as much better. It provides much less flame leakage, and much better alignment of the bullet, as well as better support.

     Some folks (including me) have had good success without bothering to slug bores by the simple expedient of trying to press the nose of a cast bullet into the muzzle of a rifle by hand. If it drops in of its own weight, it is too small for best results, no matter what diameter you may size the body. If it doesn’t drop in, but can be pushed in with finger pressure, it’s much better, but may still be too small for the very best results. If the nose can’t be pushed into the muzzle, the bullet will give you a good chance of really good results. Then size the bullet body as large as possible without causing chambering problems. It may not be sophisticated, but it’s still very practical.

Ken Mollohan

But . . . . . .

     The orthodox doctrine states that we MUST fit the bullet to the chamber, to the throat. FIT, the doctrine screams, is the real deal. Now if everybody knows that it's true, then I feel pressed to agree, and have taken measures to fit bullets to the throat/chamber of rifles since the mantra was first chanted.

     But now and then I think about the orthodox doctrine that ulcers were caused by stress, and the brave M.D. who held out for the bacteria theory and got the flak.

     And I think of the old journalism school joke:" If your mother says she loves you, check!"

     Lyman has produced molds for the 308291/311291 and 31141/311041 bullets for a long time, maybe a hundred years. In that time we've seen many 30 caliber molds come and go (remember trying to get 311413s to shoot?), but these two bullet designs have staying power.

     My 31141 mold has base bands of .312"/.313", a front band of .303"/.304", and a nose of maximum .300".

     My M54 30/30 has no throat, the case neck dimension ends with a ?45 degree? angle getting right to the rifling. I've worked with the M54 and various molds, 311299 and 314299 mostly, beagling and sizing and fiddling. The 31141, sized .312", shoots as accurately as any bullet, approaching 1" averages for five, 5-shot 100 yard groups.

     My Martini 30/30 bench gun has a .310" cylindrical throat about .150" long. I've worked with the same 311299 and 314299 molds, searching for accuracy. Without gas checks. Here's a throat-fitting bore-riding opportunity at its best. This gun has shot well, winning matches in Single Shot competition, shooting a few under .3" 100 yard groups.

     I don't shoot many 31141s in this gun, but when I do, sized .312", they shoot almost as well as any other bullet. (I think I need the long '299s for the wind. 

     Both guns shoot 308403s, that just have nothing to do with the throats, very well but at low, (7/Unique), velocities.

     These 31141 experiences aren't aberrations, we know that because the 31141 (and 311291) stay in the Lyman lineup-people are buying them.

     I don't do much with the 311291, I have a mold with the GC shank milled off that shoots well, but it's a short light bullet - that wind!

     So am I starting a heresy? No, I don't want to be burned at the stake. I am suggesting that for novices and even graybeards, bullets like the 31141 and 311291 will shoot accurately in most any 30 caliber rifle without the shooter worrying about bullet fit and throats and chamber dimensions.

 

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Warning: All technical data mentioned, especially handloading and bullet casting, reflect the limited experience of individuals using specific tools, products, equipment and components under specific conditions and circumstances not necessarily reported in the article or on this web site and over which The Los Angeles Silhouette Club (LASC), this web site or the author has no control. The above has no control over the condition of your firearms or your methods, components, tools, techniques or circumstances and disclaims all and any responsibility for any person using any data mentioned.

Always consult recognized reloading manuals.

 

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