Many years ago Buckshot, who used to post here a lot, bored 4 driving bands into a Lyman Foster slug mould to bring it up to bore diameter. I think I found that thread from way back but no pictures anymore!
So, yes it is doable but not sure what a machinist would charge. Eric at Hollow Point Molds could do it for sure:
https://www.hollowpointmold.com/band-modifications/
He will enlarge driving bands in existing bullet moulds so cutting three or four driving bands shouldn't be an issue at all. Not sur eof cost though.
Since my Lyman Foster casts at 0.705" and it seems most cast from 0.686" to about 0.705" powder coating is out unless it is VERY thick powder coating!
A couple of things you could try easily:
- Paper patching wiht heavy paper. I have done this with some success but I think the paper patch was not leaving the slugs consistently or maybe was being damaged by opeing the crimp. Accuracy improved considerable with slugs patched to bore diameter but I got fliers. I did not spend a lot of time sorting it out though.
- Blood Trail came up with a good idea and used heat shrink tubing on Lyman Foster slugs.
You may want to try paper patching or heat shrink tubing before spending money on machining. However, machining works for sure.
Longbow
PS: I should have added that precision work is required here... though I suppose not as critical as a rifle mould but still, if you want the mould bored to bore diameter or driving bands added that are bore diameter then you will want to specify whatever your bore diameter is, say 0.729" +0.001"/-0.000". Any good machinist should be able to bore to within 0.001" and I would put the tolerance on the large side so you don't get an undersize slug. A thou or two undersize likely won't hurt but it won't help either.
The shrinkage will depend on the alloy and LASC pur elead linear shrinkage is 1.13%. IIRC Wheelweights run about 0.5%.
Also, I should tell you that my Lyman Foster slug mould is miserable to cast with! The slugs stick to the core pin very badly. I would polish the core pin up ot maybe add a bit more taper and polish but I don't use that mould anymore at all. i never got good accuracy from those slugs and I hate casting with the mould so... it sits on the shelf.