Partsman-
I have a habit of picking up empty brass wherever I happen to be shooting. A few years, I was up in a canyon in Southwest Wyoming where the locals (I was one of them at the time) liked to impromptu shoot. No tables, just lots of places to stick a target or toss a pop can. I picked up a whole bunch of 7mm Rem Mag and noticed some were weirdly shaped. Bulged in places, but vaguely representing the 7mm rem mag. They were head stamped 270Win - the 3rd one I picked up was ruptured at the shoulder with traces of blood. Doesn't stop me from my experimenting but admonishes us to be careful. Larger bullets will cause higher pressures, true, but smaller stuff that allows the powder to burn faster than it is intended and pressures will skyrocket.
I have intentionally fired 257Roberts in my 8mm Mauser, just because, and I did split a few necks, but no hard extraction or other pressure signs, due to the fact that they both headspace at the same place on the shoulder and same shoulder length. The biggest head-turner in my ammo box is a box of 32WinSPCL, all trimmed to 1.285 and loaded for my 41Mag. They work. I necked down some 8mm Mauser to 257 and loaded them for the 257Roberts without issue and necked up some 257Roberts to 8mm mauser, again without issues. I later put them back in their proper sizing and all is well, save some premature neck splits due to the over-stressing in the sizer dies. I like to experiment, but also read everything I can find and get input from as many avenues as are available and above all, be careful. I have some 30-06 cases trimmed and sized for my 6.5Swede. An 8mm Mauser case got sorted into my batch of 356Win, which I found in the sizing process. I decided to proceed, just because. After full length sizing, I trimmed it to length for the 356Win, loaded it along with the rest of the 356Win, and my Big Bore 94 fed the rimless 8mm case right along with all those semi-rimmed 356Win cases without a hitch. That shot didn't spoil my group, either.
Presently working on brass shotshell cases for the 41mag and 45Colt. To me, an interesting concept that I read about in these posts. I've always made 41Mag shotshells from Elmer Keith's recipe with gaschecks and Unique powder. I'm making longer brass cases from 32WinSpcl and we'll see what happens in the 41Mag. The 45's will come from 444Marlin cases as soon as the mailman delivers them.
Regards,
Sweetwater