Guess it was about '95 I got my Colt AR in 7.62x39. Was very pleased with it's accuracy with .308 diameter bullets. Less pleased with it's blocked six round mag. I thought, what if I changed the taper of the 7.62x39 to match that of the .223. Then I could load more rounds in a straight mag . Maybe change the caliber too. .223,243,257,.270?
Never got around to doing it but surly someone did. Anyone know the results.
With a variety of cartridges available for AR15 platforms, for which others have already done the development and commercialized, I'm not sure that the view would be worth the climb. .30 AR, 6.5 Grendel, 6.8, in addition to .223.
Clarence
The x39 case with the taper blown out is available in several calibers, the first and most popular being the 6.5 Grendel which has commercial brass available. There are variants from .22 to 35 caliber, but the Grendel is the only one with dies and parts available off the shelf I am aware of. I have a reamer for a 30 Grendel Imp 40', but haven't got round to the project yet. Dies will be expensive unless I make them. Seater is easy, sizer, not so much.
I have a 7.62x39 in an A4 configuration and have had no problems with ten round C-Products mags so far.
Mine was an idea from twenty years ago. Obviously I haven't followed recent advances. I felt a reduced taper would allow increased magazine capacity/powder capacity. Might have put the x39 in the 300 Savage neighborhood, which is a very nice neighborhood to be in.