I've never had an upper in .450 Bushmaster, but did have one in .450 SOCOM. That one came from a small outfit in Virginia and had serious feeding problems. Lost interest in it and found it a new home.
Now that I live in feral hog country here in west central Texas, a big-bore upper for my AR started to make a lot of sense again, so I thought I would try the Bushmaster.
I bought mine from Bear Creek Arsenal -- 18-inch, 1:24 twist -- fully knowing that they are not the most trouble-free brand in the business, but I wanted to try a right-side charger as well and they were almost giving them away.
I've got a Vortex Strike Eagle 1-6X mounted on it and hope to break it in this week if possible. Have it well-lubed with CLP and have both Hornady factory and some handloads to try. Will report.