Help-Advise on shooting a SC 6-43 barrel that has been modified

[Chuckindenver]

breaching washers or shimstock should never be used on a squared threaded barrel... the rifles you listed, have a V thread, and the V has most of the crush...on a square thread barrel. the shoulder takes most of the crush,
Krag, 1903, P14, 1917, Garand, 1928 SMG, Winchester M54, Jap Arisaka, all have sqaure thead.
i dont recomend using shims or washers on any barrel to make it fit. there is a fix for that barrel, its a lot of work, but can be done..