16 November 2009
Like Jeff Bowles I also cast commercially. I size all my bullets one thou over groove. For the 40S&W thats 401. Bur some guns shoot as well with groove dia bullets as with one thou bigger. Sizing one thou bigger than groove makes sense in commercial casting because being slightly on the fat side will be more accurate in most guns without ant pressure problems because cast bullets are comparatively soft and squeeze down to size in the barrel easily. But note the jacketed bullets are groove dia, ie 400 for the 40 S&W.
If you are shooting two guns the same calibre why would that be a question ? Sioze 'em all 401. Unless of course you want to slug the barrels. Then, when you know the exact groove dia you can size the bullets to suit, or one thou bigger depending on what range tests tell you what works best.
Frosting and incomplete fill out are contradictory. Frosting is too hot and incomplete fill out is usually too cool. But you can get both at the same time. Big calibre cavities can leave a thin web of metal between cavities. Especially with Lyman two cavity which are quite small blocks to start with. I have some which even at 600F overheat that thin web so that the bullets are frosted at that point and the frosting cause incomplete fill out. I have one RCBS 44 Mag mould and a Lyman 454 x 325 grain Casull mould that have to be used as single cavity moulds because of that phenomenon. But I have cast fro both cavities of an identical RCBS 44 Mag mould with no trouble, which proves that moulds can behave differently.
I agree with Jeff about cleaning the mould. They leave the factory with a film of protective oil. If you don't clean it off it can burn into a hard crusty deposit that will make good casting impossible and will be difficult to clean off. Nothing fancy is needed. Just clean them with turps ot thinners. The whole mould not just the cavities.
I also agree with getting a dial caliper. That is I agree with the principle but not the choice of instrument. Dial calipers are very useful and versatile because they can measure outside, inside, length and depth. But for various reasons they are not quite as accurate as a micrometer. Every handloader should have a good one inch micrometer. Probably less expensive than a dial caliper. With the change you can buy an inexpensive vernier caliper which is a dial caliper without the expensive dial. But hey, I'm not putting down dial calipers, thay are very nice tools to have, but you'll do better with a mike and a vernier.