Because many you talk to ( some of the guys selling at shows ) seem to see alot of problems with the 7.62X39 being hard on extractors ( maybe from steel case ammo)?
not extractors but the bolts themselves are prone to breakage. the extractors are fine, but a 7.62x39 bolt simply has too much material removed from the face and is at the limit even with factory 7.62x39 ammo
Here's the problem with a 7mm AR15 variant, it may seem like a good idea but the problem is that ALL hunting weight 7mm bullets are designed with cartridges in mind that have almost 1000 fps on what's possible from the AR magwell creating problems with under expansion on game. Another consideration is the SPC case would have to be drastically shortened to allow most 7mm bullets to fit in the magwell further reducing MV's
Since the 30 Remington Case is the paresnt case for the 6.8 and the the 30 Remington is a rimless version of the 30-30 then necking up 30 Remington cases that were necked to 6.8 would make the round = to 7-30 Waters.
bear in mind that the SPC is a greatly SHORTENED 30rem variation. I shoot a wildcat that necks 6.8 cases back up to 30 cal, but it's not 30rem it's the short lil 30hettett