360 Buckhammer brass for 400 GNR Short?

Thickness x 2 plus the O.D. Of the bullets (.399-.403”) that you use. .30-30 brass varies wildly in mouth thickness and rim thickness as well! If the case is tapering in thinness, thicker cut off, it might not chamber. We’ll, without inside case reaming.

problem is solved. Lots of useless new cartridges out there. You could buy a proven cartridge like the 445 where you can shoot 445, 44 Magnum or 44 Special. Or you can follow the crowd and buy a gun like maybe the 350 Legend, or the 30 Super Carry or any of a number of new cartridges that the gun magazines rave about (because they get paid to do so). It's your baby so knock yourself out.

You only need maybe (20) pcs. of brass so making them is not too troublesome. (For working up a load and hunting) As I mentioned you may have to inside neck ream. The Starline .38-55 Win. brass can be had for a thinner brass. It might be a start of an easy fix. The Game Warden's DO carry measuring devices for straight wall case hunting! If you are nice chances are much better they will be too.

Whack some brass and just see. I made some "rimmed" 10mm cases once for a revolver once. Sort of a 10mm Auto Rim just to see if I could. But it was a lot of work as the brass was terribly thick cut off.