In an AR-15, if you could only have one the 5.56 is really the best way to go. The caliber is just TOO popular to ignore. Cost of ammunition is the other factor. While the 5.45 may be cheaper for now, once surplus ammo dries up all you'll have is Russian imported ammo. Also, as a true SHTF/defensive carbine there are some great options for the 5.56 these days.
For an alternative caliber, the .300 blackout is turning out to be the next big thing. Simply put, to change calibers all you have to do is change the barrel. It uses the 5.56 everything else. The Blackout in with 110-125ish grain loads is a modernized/more AR friendly 7.62x39. If you dabble with suppressors, there are excellent subsonic options as well. Performance out of a short barrel is also outstanding.
For an alternative caliber, the .300 blackout is turning out to be the next big thing. Simply put, to change calibers all you have to do is change the barrel. It uses the 5.56 everything else. The Blackout in with 110-125ish grain loads is a modernized/more AR friendly 7.62x39. If you dabble with suppressors, there are excellent subsonic options as well. Performance out of a short barrel is also outstanding.
