450Bushmaster.net

Can a .450 Bushmaster fire, or be converted to fire, .45acp? No, but yes. It all depends on if you have the means, ability, & time to invest into modifications. It all boils down to, not the barrel or the bolt, but rather the chamber portion of the barrel. .450bm & .45acp, both being straight walled cartridges, eliminates the ability to produce a simple casing shaped chamber adapter, which is what allows the .22 conversion bolt assemblies to function in a .223/5.56 rifle. That said, a chamber length modification would be required in order to convert from .450 to .45. Now the means, ability, & time aspect. It's my opinion that the simplest method to achieve the conversion would be to bore out & thread the chamber to accept a threaded chamber insert designed for the desired cartridge, kind of like a shotguns' choke. These chamber inserts would be made to essentially adjust the required headspace as needed for the cartridge. Of course, this is much more simple in theory than in practice, as the chamber is the portion responsible for handling the violent forces encountered when a round is fired. Any expansion of the chamber insert due to heat or pressure, and you'll play hell breaking it loose from its seat.
In conclusion, unless you're a machinist with the means, ability, time, and a couple .450 barrels you don't mind potentially destroying, I would recommend waiting for someone to produce a .45acp AR-15 upper, if someone hasn't made one already.

Just build a dedicated 45ACP upper.