I think that my 35 Rem Rimmed brass was formed from 30-40 Krag. It most likely is not the brass causing your issues...but just in case I'd be happy to send you a few formed 30-40 cases so that you can try them. Some rims are thicker then others (Mil v/s Commercial) and a full length sizing can cause the round to slip a little too far into the chamber if the rim is thin and misfires result. With heavy (thick) brass like 303 Brit. You'll most likely only have to neck size for quite a few rounds after your fire forming. I did not need to inside neck ream with the 30-40 brass but others using the 303 Brit brass did.

I sold that barrel to someone here on the forum, maybe they will chime in with their observations.

Bill