The guys are right. I am no expert by any means. But I am pretty sure you can get a 9mm-22lr-204-5.56-223-5.45x39mm-30 rem-450 bushamster. That will fit onto your ar-15 lower. I also think the 6.8 will fit the ar-15. The bigger cals like the 243-7mm-08-308win use the ar-10 lowers, they are beefed up and made for the bigger cals.