the lowers of the AR-15 and the AR10 are completly different. no AR-15 upper will work on an AR-10, or vise-versa.
if you have an AR-15, you can buy uppers in 6.8 SPC remington, 6.5 grendal, 50 beowulf, .458 (also madse by RRA) socom, 7.62x39 (or "AK" round) and many many more. what you need for a cartridge to work in the AR-15 is for the over all length of the cartridge to be no more than 2.260". there are a lot of "wildcat" rounds availble as well. (well, not really wildcats, but not the norm ) some are, 6x45 (basically a .223 case necked up to 6mm) 6mm PPC, 6mm WOA, 20 tactical, 20 practical, 21 fireball, 17 fireball, and the list goes on and on.
if you have an AR-15, you can buy uppers in 6.8 SPC remington, 6.5 grendal, 50 beowulf, .458 (also madse by RRA) socom, 7.62x39 (or "AK" round) and many many more. what you need for a cartridge to work in the AR-15 is for the over all length of the cartridge to be no more than 2.260". there are a lot of "wildcat" rounds availble as well. (well, not really wildcats, but not the norm ) some are, 6x45 (basically a .223 case necked up to 6mm) 6mm PPC, 6mm WOA, 20 tactical, 20 practical, 21 fireball, 17 fireball, and the list goes on and on.