I get this is apparently a touch subject, I had no intention of this thread turning into a discussion of the origins of the AR 15/10 - M16 etc... Just meant to the OP that the AR platform is a universal platform that has several uses andd calibers available, with little effort to switch between them.
I have a Ar15 with a 5.56 upper assy, and also a .458 upper assy, and they interchange just as they would normally with the lower reciever. So as I mentioned this is a great option for the Op as it has the 5.56 barrel chamber with capability for the larger pressures, as well as the ability to chamber the lower pressure .223.
on the plus side, was the ability to swap an upper to a .458 or even a 300 BLO. thats 4 calibers covered by the same lower reciever and trigger group.
I have a Ar15 with a 5.56 upper assy, and also a .458 upper assy, and they interchange just as they would normally with the lower reciever. So as I mentioned this is a great option for the Op as it has the 5.56 barrel chamber with capability for the larger pressures, as well as the ability to chamber the lower pressure .223.
on the plus side, was the ability to swap an upper to a .458 or even a 300 BLO. thats 4 calibers covered by the same lower reciever and trigger group.