Mold Lapping

[BABore]

First off let me say that I'm already in-process and have read all the posts and info over on Castpics.

I'm working on a Lyman #457643 mold. My 450 Marlin slugs at 0.4585". Using Either WW's or WW's + 1% tin, the bullets drop at 0.4581". The front scraper band, above the crimp groove is at 0.4595". This mold seems to like 750 degrees plus to get a good base fillout. I'm ladle casting with an RCBS dipper. Both the sprue plate and dipper hole have been opened up a bit based on recommendations from Dan at Mnt. Molds. When everything is at temp she sings. I had some promising groups, then everything went to hell. Quite a bit of leading then I started getting some tumblers. I've been using 2 coats of Alox and the loads are in the 1,100 to 1,400 fps range.

I lapped out the mold once with no real gain. I reread the Castpics post about using the screw to expand the bullet slightly and gave that a try. The bullets now drop with a base of 0.460 avg, middle bands at 0.4582, and scraper band at 0.4595. I cut my lapping bullet off just above the crimp groove.

I did notice that the bullet is slightly out of round. Close to the parting line, but not on it, measures about 0.001" bigger than when I turn it 90 degrees. If I shoot these unsized will that be a problem? Also, since the middle bands are still just a bit small, would you relap just that area?. I'm making up a 0.460, Lee type, sizing die if I get too carried away. Just in case.

[44man]

Babore, first, it is not possible to lap a mold without getting it out of round a little. The reason is the edges of the cavity will lead the lap to the side at those points. Doesn't do any real harm though. I would shoot it as is without sizing.
Next, get rid of the alox and shoot softer boolits (although you didn't say your alloy.) Using a softer alloy will make a smaller diameter boolit shoot very good. I shoot thousands of rounds a year with my 45-70 at those same speeds with black powder with home made lube and get no leading at all. If you are shooting smokeless, try Felix lube.
I do get leading with Wheel weight metal but not with a 20 to 1 mix.
It would be possible to lap a mold round if a slightly smaller boolit loaded with lapping compound was put in the mold and then close the blocks completely and keep them clamped tight. Now as it is lapped, a way has to be found to keep expanding the boolit while it is clamped in. Be very hard to do. It is when the blocks are slightly open that the out of round condition appears.