Bullet dimensions change with the alloy used, (more tin and or antimony make the bullets slightly larger) and during sizing in Lyman 450 style sizers. I've had 25:1 bullet noses expand .006" when sizing from .312" to .309".

     The ease of chambering a bore riding bullet, or of breech seating a bullet, is affected a lot by the hardness of the bullets. Soft bullets may breech seat easily, while hard bullets don't want to breech seat at all.

The hardness, alloy and sizing operations are all connected, and can operate to substantially affect bullet "fit" in the gun.

Most of the time a readily-available mold can be found that casts bullets that work well in your gun In unusual situations, we may need to do something to get a bullet that fits a particular gun. Here are some options.

     If the mold produces bullets that are too big or too small, or that have the "wrong" shape, then there are a number of ways to fix the problem.

Trade the mold

I've had good luck buying and selling molds on the Internet auction sites, searching for a mold that casts bullets with the "right" dimensions or shape. Needing a 429421 mold that cast bullets of .433" diameter or larger, I bought several on the auctions and cast bullets with each until I found a mold that met my needs; then sold the others.

Vary the alloy

Adding tin or antimony to the alloy makes the bullets bigger and harder. Reducing the amount of tin or antimony makes the bullets smaller and softer. The 3rd Lyman Cast Bullet Handbook, pg. 58, shows that a 30 caliber bullet cast of pure lead would measure .3095" in diameter and in linotype would measure .3108"- a difference of 1.3 thousandths of an inch. Then changing the alloy in and around the wheel weight area of composition would vary the diameter less. Changing alloy composition within the normal range of alloys makes very small changes in diameter.

Size bullets down in a lubricator sizer

To make bullets smaller we can size the bullet down in a Lyman or SAECO or RCBS lubricator/sizer with conventional dies, or use the taper sizing dies as supplied by Don Eagan in these lubricator/sizers. These dies taper the bullet from bore diameter to larger than groove diameter, and come in various sizes. I have used one of these dies in 22 caliber, and was able to custom size the bullet to adjust seating depth. This die worked very well for me, and increased accuracy of the .223 Remington rifle the bullets were used in.

To make bullets larger and/or change the shapes, we can "Beagle" the mold (put pieces of tape on the mating surfaces to make the bullets larger), lap the bullet mold, bump bullets in the lubricator sizer or bump the bullets in a swaging die and press. All of these are explained below.

How To Beagle Bullet Molds

(This is from John Goins, aka "Beagle", who wrote the first description of this process that I ever read.)

I suppose that at least once in our lives, every caster is plagued but an undersize mould. This fault as I call it occurs primarily with Lyman or Ideal moulds. Many times, I have searched carefully for a mould only to find it casts small bullets when I finally obtain it.

Many designs suffer from this problem but it seems that certain moulds or designs are worse than others. Among these are the 457191, the 357446, the 429360, the 41032, the 427098 and the 429434. Then there are the true old classics you obtain that were just an “ah s??t!” when cut. I have a 358429 that drops bullets at .358 X .355 and that was just a manufacturer’s problem.

All of these moulds set in my mould cabinet for years and occasionally I’d learn something and then try them again to no avail. Still too small.

     After retirement, I had time to mess some with these moulds and try to improve them.

First was lapping with valve grinding compound. A very messy and often unsuccessful project.

The second attempt was to drill and tap the open side of the pin alignment hole and put setscrews in so that the mould blocks could be held partially open. This works but the proper adjustment is hard to establish and maintain. About .003” clearance can be obtained before flashing occurs.

The third attempt was to take a 5/32” spring pin and cut 3/16” lengths off one end and drive them into these openings to hold the mould open enough to cause enlargement of the bullet. This method also works but the correct adjustment is very difficult to obtain.

About this time, I acquired an H & G #73 double cavity mould off E-Bay. Since I had 2 cavity H & G moulds, I didn’t worry about handles. When it came, my handles were too thick to fit the block cuts. I finally found that a set of Lyman 4 cavity handles would hold the blocks so that I could try the mould. I fully expected a .358” diameter bullet but instead obtained a .361” bullet. Close examination of the gap between the mould halves showed that there was a minute gap between the blocks. Removing the blocks, there was no gap so the handles were held apart exactly the correct amount to produce a nice large bullet.

I communicated this information to Orygun Mark and he drilled and tapped a set of handles to incorporate a single .002” shim between a set of SC blocks and it worked for him. We’re getting somewhere now.

I finally located a set of shim stock at ACE Hardware of .001, .002, .003 and .005” thicknesses. I cut and prepared a piece of .002” shim stock and prepared to shim a DC mould by securing it behind the mould handle screw. Won’t work as the alignment pins get in the way. Back to the drawing board.

I had a roll of .003” aluminum tape with adhesive backing. I tried that on a SC mould but the adhesive closed the vent lines on a .22 mould I was attempting to enlarge. Finally, I placed a large square of this tape below the cavity and small strips on either side. Success.

Now, back to the DCs. The same method of a large patch below the cavities and a small strip along the outside of each cavity works well and stays in place because of the adhesive. Now, the 427098 which previously ran .428” runs between .432 and .433. 

   I learned several things during this experiment.

• 1) Don’t completely block the vent lines or incomplete bullets will result.

• 2) The shim must be directly between the mould handles.  If below or above, the blocks will tilt and close. The shim must be directly between the alignment pins for the same reason.

• 3) For some reason unclear to me, bullets don’t enlarge excessively sideways even though there is a gap there.

• 4) Even if bullets are slightly elliptical, sizing rounds them if done with a new style tapered entrance sizer and accuracy is just as good as with round bullets.

• 5) Very little flashing between the mould halves occurs using this method. If being dipper poured, they will flash. If using the bottom pour and dropping the melt 3/4" to 1” into the sprue hole, no flashing occurs due to reduced melt temperature and the reduction in the pressure of the melt going into the cavity.

Update…

After several months of experimenting with enhancing moulds, I have learned a little more about it.

More uniform results can be obtained by applying tape to both halves of the mould.

Venting is not a problem as I first anticipated. Sufficient venting is provided by the gaps between the tape strips.

The best method of taping can be obtained by using strips approximately 1/8” wide. If you want a bigger body, apply strips alongside the cavity and between the cavities in the case of a double cavity mould alongside the area you wish to enlarge. These strips should be positioned so that the two strips bear on each other. This separates the mould halves a uniform amount.

This method can also be used on the larger 45 moulds designed for black powder to enlarge the nose to full diameter for use in smokeless rifles and give a better bore fit.

Under the current method of taping, I am obtaining about .003” in diameter without undue design distortion. This has been very useful in the .38, .44, 375 for the 38/55 and in the BP 45 designs in the 45/70.

Latest update. For some reason, if using only one thickness of tape in the mould, it seems to work best if placed on the side of the mould with the index pins. Haven’t quite figured this one out but that’s the way it works.

If there's any questions, contact beagle at goinsj3atgtedotnet. beagle

From Joe Brennan

I've had good luck with "beagling" molds from .22 to .44 caliber, using just one layer of tape and making the bullets about .0015" larger in diameter. Two layers of tape gives me fins on the bullets that I can't get rid of. Others report using two layers of tape successfully.

After putting the tape on the mold it takes about 50 bullets cast to get the tape squeezed down and the adhesive squeezed out. These bullets aren't any good, they have fins.

The trick to casting bullets without fins in a beagled mold is to use as little heat as possible and/or to vary the tin or antimony percentage. The more tin or antimony, the more fins-for a given amount of heat. The heat can be reduced by reducing the temperature of the lead pot, by reducing the time the sprue plate is in contact with the ladle or nozzle, by allowing the mold to cool between pours, or by cooling the mold on a wet sponge between pours. I am able to get good finless bullets with beagled molds by reducing the temperature of the pot and minimizing the time the ladle is in contact with the sprue plate.

The 22-caliber NEI mold has tape on the bottom, to make the nose bigger.

The 311299 mold has 2 pieces of tape, one on either size of the cavity. This makes all diameters bigger.

How To Lap Bullet Molds

Lapping Bullet Moulds

Ric Bowman

There are several reasons to lap bullet moulds: new moulds with machining errors and burrs, corrosion and stains, equalizing volumes of multi-cavity moulds and casting problems. Other than for casting problems, most lapping has been done for cosmetic appearance and for match bullets, made as round as possible. I think everyone loves to make beautiful shiny bullets with zero run-out, but it doesn’t make them shoot much better for most of us.

I have never been able to make moulds cast bullets larger than 0.002 inches greater than originally cut, and still keep them round, by lapping. If you need a bigger bullet, buy a mould that casts a bigger bullet as manufactured. However, lapping can make bullets drop from the cavity easier and remove baked on material from the surface of the cavity.

Lapping is not a “spur of the moment” decision, but takes some planning and forethought. The first thing you will need is ten of your best quality bullets cast and separated for each cavity you are working with. Wheel weight alloy is just fine except for polishing, and then linotype alloy is probably best, as bullets are larger, harder and more closely conform to the cavity. The three most useful lapping compounds are “Clover” brand automotive valve lapping paste in “fine” grit, Brownell’s 800-grit water based lapping paste and “JB” bore paste or “Flitz” polishing cream. (See picture)

To drive the lap, I use a small sheet metal screw that has a quarter inch head with a socket and “T” handle wrench. A small electric screw or nut driver may be used if you are careful and check your work often.

Let’s talk about mass produced aluminum moulds first. One problem is that they are cut with the mould halves compressed tightly together. No one can hold the handles together as tightly as the fixture that was used in manufacture. This is the primary cause of out-of-round bullets with these moulds. In addition, the manufacturing process doesn’t always have the sharpest tooling and best chip removal process and that leads to burrs. These burrs hold the bullet into the cavity. The other problem is that unless you are using a perfectly clean pot, ladle and alloy, every bit of dirt, slag and carbon seems to stick to the surface of the mould cavity. It not only makes ugly bullets, they are not in balance, and bullets stick to the mould making you hit the handle joint trying to get them to drop. While powdered graphite or mould preparation compound can coat the cavity, it doesn’t last all that long, besides occupying space that should be part of the bullet.

All aluminum moulds are soft and easily cut with lapping compounds. That is both their strength and their weakness. Very easy to get results but very easy to mess up and ruin also. When I get a new or a “new to me” aluminum mould, I clean it with hot water and automatic dishwater soap and a toothbrush. I scrub until clean and then hold it under the hot water tap, scrub until all the soap is gone, and about two minutes more. I dry it with a hair dryer I bought at a yard sale. Then prepare it for casting using what ever process the manufacturer suggests. When it starts casting well filled bullets separate ten good ones. Out comes the micrometer for measurements. Are they round? Now put them on an accurate scale, are they a consistent weight? Did they release from the cavity easily? Out comes the five-power loupe to look for flaws, burrs and problems. If the decision is made to lap here is how I proceed.

Remove the sprue cutting plate if possible, if not, you will have to work carefully not to let it get in the way and to make sure nothing get under it to mar the top of the mould. A bullet is placed in the cavity and a 1/16-inch drill bit is used to make a pilot hole as close to the center and as close to vertical as possible. I used to do this in a collet with a drill press, but found that doing it carefully by hand worked just as well. Do not let the bit come close to the bottom of the grooves or touch the mould in any way. It just needs to be deep enough to be the pilot for the screw. The screw needs to have a small shank and only needs about three threads deep to hold it tight. Place the bullet into its cavity and tightly close the mould. Start to screw the screw in the bullet with the socket. As it goes in it will expand the bullet. Now you will have to adjust the size of the pilot hole or shank size of the screw so that you do not deform the bullet. Increase the size of the hole until you can get the screw firmly into the bullet without expanding it too much. This is a “feel” process. You are not trying to hold the earth together, just enough to keep it straight in the bullet and rotate the lap.

Coat the lube grooves, driving bands and bore riding part of the nose with lapping compound. A volume of about half the size of a pea in plenty. Try to avoid the end of the nose. Place the bottom of the mould blocks on a flat surface such as a Formica workbench top, steel plate or square of plate glass. This will help align the mould blocks as the pins or guides will not be in full contact as they are closed. Insert the bullet into the cavity and hold it firmly together with the handles.

Now the “art” and not the “science” part of the project begins. The objective is to evenly squeeze the halves together, maintaining alignment, just tightly enough for the lapping compound to cut without jamming the bullet. I have used both a helper with an extra set of hands and small rubber bands evenly applied around the mould blocks. When turning the screw, do not apply any more downward force than necessary, just enough to hold the socket on the screw. You are not trying to push the bullet down, just make it rotate. If you squeeze the handles too tightly, the bullet will seize in the cavity. You want it to have good contact to cut evenly, but not to keep turning the screw into the bullet. Ideally about 25 complete revolutions of the “T” handled socket driver will allow you to close the mould with about the grip of a firm hand shake, or the amount of pressure you use to hold the mould close when casting. If you are using the 800-grit compound, the coarsest I ever use on aluminum, you have increased the smallest diameter of the cavity about 0.001 inch.

Clean and dry everything just as at the beginning. Closely look at the cavity, as your goal is to have between a third and a half of the circumference of the cavity showing the different surface texture of having been worked. If it has not been touched, repeat the lapping process with another new bullet lap until you have about two thirds of the surfaced worked. When you reach that point, clean again and cast bullets until you start getting good ones and measure them to your specifications. If they are round enough for you and fall reasonably easy from the aluminum mould, it is time to polish. If not to your satisfaction, use another newly made lap. If you are trying to make the cavity larger in diameter, you must be careful. As the cavity gets bigger, you cannot keep the lap perfectly aligned with the mould and bullets will begin sticking or loosing concentricity. If they meet your dimensional needs, it is time to polish.

Polishing is to remove burrs and improve the surface finish of the cavity. Make a new lap and coat it with your polishing compound. JB is commonly used because most shooters already have it for bore maintenance, but others prefer Flitz because it is faster acting. The process is the same as using coarser grit but it takes about 100 revolutions to get an acceptable finish. This is where a small battery powered screwdriver comes in handy because you can check your work after five seconds of rotation. You do not have to make a new lap for polishing, but you must insure that the face of mould is clean so that the halves meet correctly. Laps made from linotype last longer and may be used several times. However if you have increased the diameter of the cavity very much, you will need a lap made from its new bigger size. Again, clean and dry your mould and test for bullet quality.

     (A quick note on bronze moulds. All modern bronze moulds are made by custom and semi-custom makers. The nature of the material and the quality of manufacture are so good that only once have I ever had to work on a bronze mould. That was to remove one small mark on the center flat of a flat nosed bullet. Once you have used one of these jewels, you will know why so they are so popular.)

     Ferrous metals are the traditional materials for moulds and have one great advantage; they are hard, tough and will take more abuse than softer metals. However, they have one large fault, they rust, at times in as little as six hours! To work, they have to be oil free and that leaves them vulnerable to oxidation. I feel the leading causes of failure of the bullet to release from the cavity are burrs and “staining” or shallow oxidation of the surface of the cavity. These are easily removed with lapping. When iron based moulds rust they are both pitted and the surface of the cavity is raised by the creation of ferrous oxide that occupies a larger volume than the under laying material. Even if we remove the bump caused by rusting, a pit will form when the rust falls away from the surface metal. This pit tends to lock the newly cast bullet into the mould and makes bullet removal difficult and lapping can only ease the problem and not remove it.

Let’s look at an iron mould that we are trying to save. Prepare it for casting and make the ten of the best bullets possible. While it is cooling, prepare your first lap. This is where the Clover fine grit comes into play. It is about 340 to 400 grit and it will imbed into your lap. Using your first lap, rotate it about 50 revolutions. Clean the mould in petroleum based solvent like isopropyl alcohol. Closely examine the cavity through your magnifier. If not all of the red colored rust is gone use another lap of the Clover compound. If only light staining is left, you can now proceed to using an 800-grit compound and a new lap. Fifty revolutions will normally even out the cavity surface. Then the polishing compound, another new lap, should get you to a workable finish. To get the best possible finish, you will have to cast a couple of new bullets to use for the final polishing laps.

How many revolutions, or how long do you rotate the lap? This is in the “art” and not in the “science” category, unless you want to make the calculations yourself, as it is to complex for me. Here are the factors:

     The larger the diameter of the lap (bullet) the faster the outside surface is going at a set number of revolutions. A 45-caliber lap cuts faster than a 30-caliber lap.

• The coarser the grit the faster it cuts.

• The more pressure closing the mould haves, the faster the cutting takes place.

• The harder or larger the lap, the faster the cutting takes place.

• An excess of compound does NOT make the lap cut faster. The lap will only imbed so much, and the rest is thrown out by centrifugal force.

The first 30-caliber iron mould I ever lapped I used medium grit Clover valve grinding compound and a quarter inch drill motor. I squeezed as hard as possible without running the screw deeper into the wheel weight alloy lap. The motor ran exactly 30 seconds. What a mess when the next bullet was cast from that mould! The nose was 0.0015 larger on one side and the bottom driving band was 0.003 larger on the opposite side. The mould block had to be hit with a mallet to release the bullet. Too fast of rotational speed, too coarse of grit, and lack of control with the drill motor set up some bad vibrations making the lap wobble in the cavity. Since then, I have learned that lapping with a “T” handle driver or low speed battery screwdriver is less error prone.

Picture 2 shows a used wheel weight lap charged with JB compound that has been turned 100 revolutions in a Hensley and Gibbs #119 30-caliber mould to clean out 50 plus years of crud, rust and surface staining. The scrap lead bullet cast from the lapped mould shows acceptable surface finish and falls easily from the cavity.

While I only use single cavity moulds, or one cavity from a two-cavity mould, to make match bullets, others don’t like to do that. Moulds cut with cherries do not always have identical cavities due to the manufacturer not insuring that there are no chips in the cutting fluid and moulds faces meet perfectly at the final cutting. Once you develop some skill at lapping, you can equalize cavities. This is done by alternating laps made from front and back cavities. It is a lot of work, but if careful you can get cavities to cast bullets within 0.1% of average weight over a sample of 10 bullets from each cavity.

If you have the ability to cast decent bullets, you have the ability to lap moulds. Think about what you are trying to accomplish and take your time. Remember it is a lot easier to take more metal out than trying to put some back.

Three Reasons to Lap a Mold

John Wagner

There are three reasons to lap a bullet mold.

No. 1 is to enlarge the mold or increase the size of the drive bands or the nose (bore riding) part of the bullet.

No. 2 is to fix a stubborn mold where the bullet sticks in one half of the mold.

No. 3 is to correct a mold that is out of round. This can also be a cause of a stubborn mold.

No. 2&3 are the best lapped out by hand with the use of a tap wrench or allen wrench.

No. 1, to enlarge a mold or parts of a mold can be done best with a drill press or lathe on the slowest speed possible. If your lap runs true, with no wobble, then you can use slow speed turning and do the job faster.

 If you try to lap a mold with an out-of-true lap with motor power, then you run the risk of making the cavity out-of-round. You don't want this.

To lap by hand I use a tap wrench. I use fine valve grinding compound mixed with oil or mineral sprits, remember that oil will contaminate the mold and must be cleaned out with mineral spirits. Mineral sprits will not contaminate a mold.

If you want to store a mold for a lengthy time use a fine sprays of WD 40, inside and out. Molds will not rust at all. When you are ready to cast just rinse in the mineral sprites and wipe off with paper towels. All the spirits will evaporate when the mold warms up, the first bullet will be wrinkled but the second bullet will come out fine, this is after preheating the mold before casting.

When you start your lap turn the lap by hand, forwards and then backwards, until the lap turns freely with a little pressure on the handles. Then wipe the excess lapping compound off the blocks and they will close tighter this time.

Sometimes the lap will stick tight, if this happens you might screw the tap or screw in to the lap deeper and it may enlarge it or at worst the tap or screw might come out the nose. This is Bad News. To prevent this happening just put a small bushing over the top of the lap up to the jaws of the chuck. This will prevent the tap or screw from going deeper into the lap. The bushing can be made out of a piece of copper tubing or a drilled out 22RF case or other small pistol case, as long as it is smaller than the lap.

To lap aluminum and brass molds you can use any cleaners for lapping compounds. Such as J.B. bore cleaner, Comet, Bonami, Zud and even toothpaste. It cuts slower and smoother and leaves a smoother polished cavity.

On iron molds you will see white spots form on the cavity. This means your lap is cutting these areas; otherwise the cavity will remain blue where it's not cutting. After you are done lapping the mold must be reheat treated in a blue color to cast good bullets. You must run the lap to enlarge the cavity until you see the white iron on the complete circumference of the cavity or your bullet will be out of round.

Remember your laps should be cast of hard alloy like linotype or stereotype metal, this will fill out the mold better and shrink less. It is the best to let the lap harden 24 hours to stress.

Relieve it let the lap reach maximum hardness before drilling or chucking in a lathe or drill press chuck. The light chuck marks on the bullet are okay to hold a little more lapping compound.

If you do not want to lap out the grease grooves on the bullet just take a knife or a needle nose file to the grooves on the lap so it won't lap out the sides and bottoms of the grooves. 

If you want to enlarge a mold more then .002"- 2 thousands of an inch, then you will have to cast more laps of the cavity as it gets larger, you will have to clean the mold out good before getting a good bullet to use as a lap.

To use an allen wrench as a turning handle, just drill a hole lightly smaller then the allen wrench and drive the long end of the allen wrench into the hole with the lap held tightly in the mold and the blocks in a padded vise.

Getting the mold ready to lap and preparing a new mold

Joe Weist

• 1. With a small file de-burr around the edges of the sprue plate and polish with crocus cloth.

• 2. If the sprue plate smears lead, take it off the mold and on a glass surface with fine wet and dry paper clean and polish it and check for flatness.

• 3. Rub moly powder or graphite into the sprue plate.

• 4. If the mold leaves a bump or a high cut off on the bullet, find a drill bit just a few thousands larger then the cutoff hole in the spruce plate and drill straight through it. Doesn’t counter bore the larger hole? This will make the cutoff hole a few thousands of an inch bigger and make the sprue plate cut better.

• 5. If the tops of the blocks have lead smears and looks rough, polish them with a fine stone and treat it the same as the bottom of the sprue plate. It helps if you can take the sprue plate stop pin out.

• 6. Now de-burr the top of the mold and put about a 1/32" round corner break on all the   edges of the blocks. Then using a single edge razor blade, clean out the vent lines going from inside to the outer edge without touching the edge of the cavities.

• 7. With your fingernail, check the inside of the mold to see if there are any sharp edges. With a new single edge razor blade, run it around the cavity to de-burr it. It doesn't take much pressure to get the burr off. You don't want any chamfer on the edge of the blocks, Check where the gas check or base is as this can be a stubborn place.

• 8. Check your alignment pins in the blocks. Wiggle them and check for movement. Adjust the pins as needed. I hold one block horizontal and the other vertical. Place the line up pin in the mate’s hole as when the blocks are in the correct position. I have found blocks with loose pins doing this. This must be done before lapping and casting.

• 9. Now degrease the mold with Tech Carburetor cleaner and scrub it with dish soap, hot water and a toothbrush. Put some moly powder on the pins.

• 10. Preheat the mold on an electric hot plate while the lead is melting. You can start casting immediately.

• 11. Cast a few times and if the cavity isn't dropping the bullet within 10-15 times you will have to lap it as somewhere it has a burr in it or could be somewhat out of round.

Lapping The Mold

Joe Weist

For bullets 35 Cal. and under I drill and tap them 8X32 and leave the tap in the bullet. I have an Ultimate mini lathe with no run out.

To lap a cavity that the bullet is sticking in, I cast a bullet of linotype and with a fine file I cut two fine lines down opposite sides of the bullet. I make a paste of light oil and a pink polishing paste called Brite Metal for household items. I also have some very fine lapping compounds from a machine shop, one is white and other is green. I have used this with great success for a mold not dropping the bullets out with more then two taps. Put the lapping mixture on the lap. Place the lap into one of the blocks, fit the other block to it, and turn the lap a little with a tap handle on the tap. Take it apart and clean the compound off the halves completely. Then put the lap in again and turn some more. You will have to try this a few times before you can get the blocks to close tight. Turn a few times until it turns freely and check inside for spots that didn't clean up and apply a little more paste. Clean up the blocks good, preheat the mold and cast some bullets and see how they fall out of the cavity. Most of the time I have found that is enough for a mold that wouldn't release a bullet. You don't want to do a lot this way as you can change your gas check size. Turn the shank down smaller. Lee molds work great lapping a burr out like this.

To make a mold larger or fix an out-of-round mold, make the mold bigger on the bands or bore ride nose

Cast two good bullets from linotype. With the bullet in the lathe or drill press, file some material off the areas you don't want bigger. The gas check shank will be one for sure and the grease grooves also. If you just want the bands bigger also file the bore ride section of the bullet.

With your file cut it two or three fine lines and apply your lapping compound. You may want to use a coarse grit, I have used valve-grinding compound and find it too coarse. Sometimes ok in an iron mold, but not on brass or aluminum. With tap wrench on the tap, fit the blocks together and start lapping.

De-burr the mold because of sticky bullets

Put the lap in the mold with the halves tight together. Turn the lap a dozen times and add more paste as needed. Clean the mold and cast bullets now and then, to measure progress. If the bullets are undersize, then lap some more. If the lap is running true, no run out or wobble, I chuck it up in my bigger lathe and run it in neutral gears very slowly and lap it out, a drill press would work the same way. I had run out once on a RCBS 165 gr. Silhouette. bullet mold, lapping in the lathe. I felt a little wobble in the lap, and when I was done the mold had .003" run out and when the bullet was sized or bumped it never would shoot again. I needed a set of blocks to be recut to 35 Cal. so the blocks were salvaged. When opening up a double cavity mold, lap one cavity to size and use the bullet from the lapped cavity to open up the other cavity, working your way up in size. I have spent eight hours increasing a mold .003".

To do this story I took an old mold lying around for forty years. I stoned underneath the sprue plate and polished it. The same on top of the blocks and the face of the blocks. There was an old bullet in it, and I used it for the lap. It cleaned the cavity up all around. When I cleaned up the outside of the blocks it turned out to be a Winchester mold in .45-70 350 gr. Now it looks like a new mold inside.