I'm kinda new to the AR-15/platform & am looking into a bigger caliber(than 223/5.56) upper receiver for deer/hogs/zombies... So far, my limited research shows the 6.8SPC is the up & coming caliber for the AR-15 platform(for heavier game)...My question, and I'm no ballistics expert(even in my mind) is why not use/develop the 7.62X39?? It's one of the most available calibers ANYWHERE in the world,shoots 308 cal. bullets, & brass/bullets should be much cheaper, right?? I'd like to stay lightweight(~6 1/2-7lbs) w/ this gun, so I think the AR-10 platform is out for me.. Thoughts/Opinions?