The AR-15 as it stands right now is probably one of the worst weapons in the entire game, not only does it have probably the slowest fire rate of any weapon in the game, it also does only 5 more damage than the G23, the AR-15 is meant for long range encounters however it still mostly sucks at that and I cannot really think of any reason to use it over the G23 other than the ammo.
I think some ways of balancing the AR-15 would be giving it higher damage, such as 25 or 30, this would make sense because of how slow it fires overall, or just simply make it shoot faster.
Compared to the M870, which is great at close range combat, the AR-15 is crap and only has some sort of use if the target is a far distance away.