Hi there. I'm in the process of forming some .348 Win cases for .50-70. I'm working with 10 cases as a test run. First off, I think I got a bit too eager because I forgot to anneal them. But, other than that, my process was the same as I did with my Swiss Vetterli with Lebel cases: a few grains of pistol powder and the rest full of corn meal, capped with wax, and fired upward to keep the powder by the primer. I did not trim any length off of them before fire forming. This worked fine in my Vetterli.
Every single case came out asymmetrical. That is, on the side that faced downward in the chamber, the wall of the case blew out straight/correct. But on the opposite side, the .75" or so of wall that's towards the rim didn't blow out all the way, but as you get closer to the mouth, it does. I know that the chamber itself is symmetrical because I cast it with Cerrosafe.
The only thing I can come up with is that the block is allowing for uneven head clearance, so the case gets tilted when fired. Though that doesn't explain why the mouth (last half inch or so) of the case seems to blow out more normally. I was able to run them through a full length sizer afterwards, but obviously it can't push the walls out. I even took one and repeated the fire-forming process but turned the case 180 degrees, only to have it come out looking just as it went in.
I'm sure there's something I'm missing here but can't think of what it is. My "screw it" mindset almost let me do a nearly full load of BP with a real boolit just to force it to form the rest of the way, but thought that I'd better ask here first.