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 shit!' 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 set screws 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 a H & G #73 double cavity mould off E-Bay. Since I had 2 cav 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. Wont 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 leaned 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 ¾. 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 is the way it works.
If there's any questions, contact beagle at goinsj3atgtedotnet
Beagle