I am making a gas check cutter & punch to make my own checks. I need to know the ID of the cup on .375 & .452 gas checks so I can make a punch to punch the blanks into finished cups.

Doug, I just "miked" a Hornady check with my Lyman dial caliper, and the ID measured .417". But I am not so sure that is the best way to go about this... I am thinking the better way might be to mike the gas check shank on a GC bullet and size the cup to a couple thousanths less than that. Your thoughts?

I have an H&G #46 GC mould around here somewhere, and I sometimes leave the cavities full when I finish casting. If that is the case I will drop the bullets and mike the shanks. (And FWIW, the Hornady checks fit the "as cast" diameter of the H&Gs like a glove with my alloy ...)

Well, the cavities were left empty, but as best I can tell the ID of the mould at the GC shank cut is .415". I don't have a set of inside mikes, so take that into consideration. But maybe close enough? I think I would use .413" -.415", gamble on a little bit of "spring" in the gas check, and go... .417" might be just big enough to slip off the bullet shank, and that is very frustrating to me.

First you need to look at what you are sizing your bullet to and the material you are going to make the check from.
Have a look at the 2nd post on this CB thread for the general sizes.

http://castboolits.gunloads.co...Checks-vs-Shank-Size

And this might help as well. Add ID of check to depth x2 and make the disc that size. The cup punch needs a slight taper so the check will come off easily and I also found having the die to punch the cup in a little larger as well to save extruding the walls of the check and made it easier to slip onto the GC shank of the bullet.The checks from my 7mm set are just as effective on 6.5 bullets, although I do use a double layer of .012 for the 7mm and a single layer for my 6.5.

Thanks guys! You always come through.