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Chamber Throats 101

By: Mike Bellm
     First, consider what happens to that perfect (as perfect as the bullet makers make 'em), shiny bullet when you pull the trigger. It leaves the case, meets the rifling at the nose end of the bullet, and the rifling engrave into the bullet as it passes down the barrel. While the nose of the bullet centers itself in the tapered ends (leade) of the rifling, the butt end of the bullet is centered only as completely as the tolerances of the cylindrical section (if there is one) of the throat and the chamber neck allow.

     In essence the throat is like a forming die in as much as the bullet takes on a different shape than it had when it left the manufacturer's plant, or your cast bullet mold and sizer. It is assumed, naturally, that the bullet was made in precision, close tolerance forming dies where tolerances are kept down to the 1/10ths of a thousandth.

     The forming die in your barrel's chamber, the throat, is the only dimension in the chamber that allows being held to extremely close tolerances, simply because the bullet is the only component in the system that is held to very close tolerances.

What Kind of Tolerances Are Typically in Chambers and Throats?

     Typical reamers from the reamer companies, one in particular, as standard are .001" larger than bullet diameter for that given caliber. Back when when I was still buying reamers, I would always pay the extra fare for them to take their throat reamer off the shelf back to the grinder and spin usually .001" off the reamer's diameter to get it down to within a 10th or so of bullet dia. for the given caliber.

     SAAMI specs. for chamber throats are commonly .001" larger than bullet size, while some are as much as .005" larger than the bullet. No Joke!

What is a shot out barrel?

     One that has had a lot of rounds run through it, eroding the throat. Did the throat get smaller in diameter? No, it got larger, enough larger that it no longer guides and supports the bullet as it engraves into the rifling.

     It allows the base of the bullet to deviate off the axis of the bore and go into the rifling at random angles, ie., cockeyed, distorting the bullet and throwing it out of balance. Remember, some bullets are spinning at over 200, 000 (yep, count the zeroes) RPM. You know what a little imbalance does to a tire on your car, and it sure isn't spinning anything like this!

The Effects of Bullet "Cant" on Accuracy

     Harold Vaughn in his book, Rifle Accuracy Facts, studied the effects of bullet "cant" extensively and verified that how the bullet enters the rifling has a very dramatic and predictable effect on accuracy. This is excellent reading by a foremost and nationally recognized ballistics scientist..... not a "magazine expert" who long ago sold his soul to magazine advertisers and can't tell the truth for fear of offending an advertiser.

     Harold also discusses throat diameter and alignment with the bore and states that nearly every factory chamber he has studied was deficient in this regard. What I find most interesting is that I learned this by hands on experience only, but as a scientist Harold put the hard numbers to the experiments he performed and validates what I preach about chamber throats. Contact Precision Shooting Magazine for a copy, or email me.

What about the chamber neck helping guide the bullet?

     Chamber necks, due to variations in brass thickness, usually have to have something on the order of at least .003" clearance to be reasonably safe---we're not talking about turning case necks to match tight necked chambers, another story. Typical neck clearances are more like .005 to .007," so the chamber neck and case neck really can't do anything positive. They can hold the base of the bullet pointed in the general direction, but the forces exerted on the bullet are far greater than a .012-.015" thick brass case neck can overcome!

     Only the steel in the chamber throat can positively align the bullet, yet with the tolerances you typically find in most all factory barrels and the majority of custom barrels chambered with off-the-shelf reamers the throat will be substantially larger than bullet diameter---the same condition as a shot out barrel!

     Unless you specify throat diameter, much of the time you are buying a barrel that is to a significant degree "shot out" before you fire the first shot through it.

     New reamers will likely be the worst, older reamers that have the throat section worn down will be closer to size.

     Stop and think about this. You read all kinds of stuff about seating depth, overall cartridge length, seating the bullet into the rifling, or backed off so many thousandths from the rifling, but tell me this, how many times have you read or heard anything about THROAT DIAMETER ?????? I don't waste much time reading magazines any more. Maybe times have changed, but I have seen throat diameter mentioned only a very few times in all my 52 years, 47 of which reading magazines.

Side Note:

     (I co-authored the chapter on chamber throats in the 1995 Precision Shooting Annual. In that same publication was a chapter about barrel lapping in which the author MONITORED THE GROWTH OF THE THROAT DIAMETER in the process of fire lapping his barrel. If your throat is too large to start with and you fire lap the barrel, you WILL end up with a still larger throat DIAMETER. I want you to burn DIAMETER into your mind. Give second thoughts to fire-lapping a barrel without re-chambering it to a longer cartridge to get into fresh rifling with a new throat.)

     I take all of this into account, when I re-chamber a barrel and use techniques and tolerances that give the bullet its best opportunity to slip straight down the tube with the least distortion.

Ok, So what do you have in any given barrel?

     Sit tight. Follow along. I want to guide you into a mindset that will open doors for you. It will reveal some of the mysteries of the chamber throat. When you get done with reading this, you will know what you are looking at when you peer down the breech end of your barrels.

     Contenders and Encores are great for looking into. With the barrel off the frame you don't have to look very far into the chamber to see the rifling. And you can readily roll it around and examine the circumferences of the various cuts at the neck and throat.

     Go pick up a barrel. Larger bore sizes are better to start with, easier to see details. You need a light source, but not too much light. In a well lighted room, looking through the barrel at a piece of white paper works very well.

     Ok. You are looking down into the chamber, looking right at the ends of the rifling. Between you and the rifling is a big dark hole. Between the big dark hole and the rifling is the throat. It is the area the same as the grooves in the barrel, minus the rifling. The throat is merely the area in the barrel where the rifling are cut away so the bullet can stick out of the case.

     In case you picked up a .22 Long Rifle, technically .22 LR does not have a throat since the bullet is the same approx. dia. as the brass case, thus the chamber is cut the same diameter all the way from the rim to a point past the bullet. This diameter, by the way, will be anywhere from .001" to .008" larger than bullet diameter!

You can check for these basic elements:

     1) Relative length of the throat

     2) The geometry of the throat

     3) The DIAMETER of the throat, relative to the groove diameter of the barrel

     4) The alignment of the throat with the bore, or more correctly the grooves of the barrel.

     Quickly, you can see if the throat looks long compared to the throat in other barrels, or if it is short. Some are so short as to be virtually non-existent, but rather a chamfer on the ends of the rifling.

     Geometry of the throat. Is it a cylinder where the riflings are cut away, or is it a cone. Most of the more recent factory barrels in .357 Mag, .357 Maximum, and .44 Magnum have a cone approximately .4" long. If you look down the wall of the chamber you can see where it begins to change from chamber wall to a long taper that ends on the tops of the rifling. What you want is a cylinder that will support a significant length of the bullet's shank as it engraves. The only tapered part of the throat should be just on the ends of the rifling.

     Remember what I was saying about the base of the bullet? The case mouth can't hold it in alignment, and in the .357's for example, the base of the bullet can move about .012" in any direction off the centerline of the barrel. So what does it do when it meets the rifling? It has every chance in the world to go in cockeyed.

     Ok, now we're getting down to the more critical part. Look between the ends of the rifling. If you see a line connecting the ends of the rifling, it means that the throat portion of the reamer was larger than the groove diameter of the barrel, and this diameter can be a thousandth or more larger than bullet diameter. For years the .44 Mag factory barrels had .440" groove diameters! And the eight groove .35 cal. barrels run as small as .356." So you really can't determine the actual diameter of the throat, but you can determine a size relationship between the two.

     If you slug the barrel with a dead soft lead slug and mike it, you can get a better idea of the throat diameter. There are other methods to measure it as well, but we won't go into that here. I simply want to point out the relationships you are looking at.

     If for example you slug a .30 cal. barrel and it mikes a true .308" diameter, then find a line connecting the ends of the rifling in the throat, think about it, the throat is larger than .308" in diameter.

     The ideal is to have the correct groove diameter, .308" in our example, and the rifling cut away with little or no discernable line connecting the ends of the rifling, meaning the throat is matched to the bullet diameter, .308."

     You usually will find a line connecting the ends of the rifling. Move the barrel around so that you can follow the entire circumference of this line connecting the ends of the rifling. Does it look the same all the way around? Or is it fainter or non-existent in part of its circumference, and pronounced on the other part.

     To the extent it does not appear uniform all the way around, it is off center from the grooves. If it is off center, how does the bullet get a straight shot in?

     Barrels that are more difficult to see the throat: .22 Mag, usually a short abrupt cone and most of the .22 center-fires where the short abrupt throats are also larger than groove diameter and hard to see.

     If you have any connections at a hospital, get one of their laproscopes and stick it down into the throat area. You'll REALLY be able to see what I have described here. It'll blow you away.

     The quality of the throat, based on the 4 criteria I listed earlier, can spell the difference between a barrel that shoots well and one you dump all kinds of money into trying to make it shoot, then "take it on the chin" when you get rid of it hoping the next barrel will be a good one.

     There are other factors to look at also, such as the crown, but we are limiting this discussion to just the throat.

     If you have a problem barrel, what you find in the throat may determine whether you continue working with it or cut your losses.

     Save yourself some money and grief. Start paying close attention to the throat. While a borescope helps a lot, you can do a lot for yourself with the "naked eye" now that you know what you are looking for.

- Mike Bellm

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This article reprinted with permission of Mike Bellm and
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.