I don’t know all of the facts and figures, but given just what I see every day on the New York and Ohio firing ranges around here, I would say that the AR15 is the most popular rifle in America.   Now that caliber changes are as easy as swapping out your AR15 upper receiver and possible only a minor change in your lower receiver like a hammer spring, you can go from a .223 varmint rifle to a 6.8 SPC rifle, or down to a CQB rifle at almost the pop of two pins.   I was never a big fan of upping the caliber on a AR15 rifle, but I am a big fan of the 75gr HPBT for close up fighting.   The 75gr bullet seems to yaw better and slower velocities and give more thump just from my experiences.

When it comes to taking your AR15 rifle to a longer range, there are records being broken every week with the AR15 platform.  People use to laugh when they would hear of some one shooting an AR15 at 1000yds,  but it is done and can give a .308 a run for it’s money.   The real thing to consider when it comes to caliber changes on AR15 upper receivers  is how powerful is the round vs just accuracy competitions.   I have a friend that came back from service over in the Middle East and he gave first hand accounts of how bad 9mm ammo was in combat and that at least 3rds of M855 was necessary to take down a threat with an M4.   Consider the ammunition you are using and not just how far out you can shoot, you may want to trade accuracy for energy.