I did a little case forming and found that a .440 base tapered to a .430 shoulder @ 30 degrees will yeild a case capacity of 33 grains H2O to the shoulder. This is good for 2772 and 2530 fps with 125 and 150 gr bullets, respectively (50,000 CUP).
Seating would be to the base of the neck in both cases, so we need a slightly longer COL for the 150 grainer. They line up nice and straight in my AR 20 round magazine, which will take a COL of 2.28, so it all works out on paper. Cases could be fire formed from factory 7.62X39 brass ammo.
I have worked with the factory case and .308/224 sabots, getting 3450 fps with 55 gr .224s, and it is very accurate in the CZ, while improving the accuracy of my SKS. The AR 310 should easily do 3600 fps with the .224 sabots and 55 gr bullets. It also shoots .312 pistol bullets with devastating terminal effects.
So, what's not to like?
I think I can make this exact chamber by running a 6MM PPC reamer .044 inches deeper than spec into my CZ chamber to move the shoulder of the PPC forward to match the current location of the 7.62X39. The throat would remain untouched, except for polishing.
Can a good gunsmith do this?
If its risky, I might be better off getting a 7.62X39 Encore bbl for test work...