I went the 450 Bushy route by purchasing the barrel assembly from MGI.
Things did not go well. I asked them to provide a bolt, gas block, flash hider, and gas tube to go along with the barrel so I would have everything needed to assemble and drop into my MGI upper. They told me the barrel and bolt were in stock but they did not have the gas block, flash hider or gas tube but were expecting delivery of the missing items in a few days. After a few weeks and a few phone calls I received my package less the flash hider. First problem, they sent me a .960" gas block and the barrel requires a .750" gas block. You would think they would have assembled at least one barrel and should have known what size gas block!!! I had a Larue .750" gas block on hand so I used my own and returned the larger gas block for credit. The barrel assembly went together without a problem and fit nicely into my MGI upper. Dry cycled a round only to find the ejection port was to small to allow the 450 Bushy round to eject. Called MGI and explained the problem. I was told to send the upper back to have the ejection port opened and I should have it back in a couple of weeks. Again, you think they would have known the ejection port was too small if they had ever assembled one of these and attemted to test fire the assembly??? I have access to a milling machine so I opted to open the ejection port myself, which I did. I have yet to receive the muzzel device so I contacted Bushmaster and ordered one from them direct. Again, I don't understand why it takes so long to get a flash hider. Bushmaster even explained to me that the thread pitch is 11/16"-24. Yes, it is finally together and it functions great even without the muzzel device. I put at least 60 rounds of factory and personal reloads without a problem. I love the caliber. If you go the MGI route, I hope you have better luck then me.
haveGot an email from Bushmaster today that my muzzel device has shipped. Finally!!