Forming cases

[Old Bob]

Additional information on 220 russian neck diameter for what it is worth. Prior to fire forming the neck diameter with a seated bullet is ranging from .274 to .275. looks like neck turning to uniform neck wall thickness & provide clearance will be advisable.

I've also resized 220 Russian brass in my Hornady dies. Prior to that I used a flash-hole uniforming tool while the brass was still 22 caliber. It changed the flash-hole size to .076" which is still a hair smaller than the standard .080/.082 that most small rifle primed brass uses. Anyway, the de-capping pin (.070") in my dies will fit through that hole.

With seated bullets (Nosler 55gr Varmageddon), the necks on my brass are .270/.271". Still plenty of room to fit in a perfect SAAMI ARC chamber. There's a good chance your loaded brass might still chamber but I understand why you would want to neck turn your case necks since the SAAMI spec for the neck in the chamber drawing is .275" +.002". Your margin is kinda thin!

Out of curiosity, I checked all 3 flavors of Hornady 6mm ARC ammo & found the widest neck diameter of all the cartridges I looked at was .2695".