The .450 Bushmaster offers a very flat trajectory out to two hundred yards or so. It is also fully compatible with an existing 5.56 AR-15 lower. All that you need is a newupper receiver, which includes a new barrel. It will also work in 5.56 magazines — a ten-round 5.56 AR-15 mag...
Now for the bad news (...Which is minor...)... Ya need the ejection port milled out a bit on your upper if ya use a regular AR upper, or ya can just buy a pre milled one from AERO, if ya can find one. Ya also need a 450 Bushmaster bolt as well. The bushmaster cartridge ...