From a source somewhere......
1. Size a 7.62X39 case as far as possible in a 41 Mag. die stripped of its decapping assembly. You will NOT be able to size the solid head, so stop when hard resistance is felt or you'll crack the carbide ring.
2. The case will now look a bit like a little belted mag case. Lathe off the "belt", but do not touch the rim. Removing the belt leaves the case head the right diameter, and the case is now semi-rimmed. I have managed the trick without a lathe just to see how much trouble it would be...chucked in a drill leaving just the belt outside the chuck, using a small file, it's not too hard to get the belt off.
3. Fire form the cases (use the corn meal "blank" method of fire forming). The cases will pop out dead straight, semi-rimmed, and just a hair over 1.5"...just enough extra to trim the mouths to even at 1.5"