FWIW the AR is fundamentally a platform, although in its pure form it does refer to the "underpowered" variant. Hunters might chamber 6.5 grendel, which is a nice intermediate cartridge commonly used to take down larger deer and elk. People who own firearms love the AR in part because it is so easy to change configuration and even caliber like legos. There are even AR chambered in .50 cal (but not that .50 cal).