I think it's perfectly reasonable to get a .300 AAC BLK kit for an AR-15--all one needs to go from 5.56 to .300 AAC BLK is a new barrel. Then, you do get a significant step up in terms of power (especially at "close" range--less than 300 yards) without paying a lot more money. I can't think of any drawbacks to the .300 AAC BLK that would affect the shooting experience of an average American, under average conditions.