I had a similar problem with 16ga RMC.
The problem originated when moving fired shells between guns i discovered the different diameter chambers.
The RMC have very thick walled shells and are a bear to resize, as have found out.
My solution was to get a big press for 50 cal BMG for maximum force. I tried annealing by heating the shells to about 600-650 degrees in a green egg. I had minor success, but ultimately i tore up the rims on more than a few and gave up.
I now segregate the wider brass shells for particular guns and stamp them for the appropriate gun. Ultimately if you have only one gun you use them in or are able to resize all chambers to match (enough metal) then it works. If not segregate.
The issue did give me an appreciation of chamber diameter differences. It also drove more to paper hulls overall when I started getting into 2.5 inch 12ga, though I do maintain one batch ofRMC for my Boss.
I never tried lathing them down the several thousandths required.
