Need a little help determining cavity diameter

[Boomshakalaka]

I usually just buy my molds, since they don't normally cost a whole lot. But my friend asked me nicely, if I could make one for his new rifle, for powdercoating.


I have measured the bore on his rifle to be 0.4582" and will be aiming for 0.46". I have already machined an old Lee sizer to .46".



My question is, what diameter should I make the cavity? Bear in mind, that he will be using the BBPC technique.

[Yodogsandman]

It will also depend on the alloy expected to be used. BBPC will add .001"-.002".

[Boomshakalaka]

The alloy will be pure roofing lead.

[Yodogsandman]

You could determine your shrinkage by drilling a hole in a donor block, pour some lead in it and measure the resulting slug. Increase the drill size until you have the right size, which allows for the shrinkage that you want. That measurement minus any allowances for the powder coating thickness.

[Boomshakalaka]

It's a little extra work, but it is probably the best solution, for the best result.

Thanks.

[country gent]

I would recomend getting the mould to .002-.003 undersize and bringing to final size lapping and casting. More work but the extra hand work results in a tryer rounder boolit, better release of the boolits, and a nicer finished surface. A small angle on the lube grooves helps release also.