I recently bought a 450 Bushmaster upper and love it so far. I got my first kill with it yesterday and the doe dropped right in her tracks..
IIRC - They 450 B was built to try and match the 45-70 load..But the cartridge had to be shortened it so it would fit in the standard AR magazine.
Bushmaster has/had the uppers on sale for $560ish this month - which is a much better deal that I got when I ordered in October from Cabales..
The standard commercial (and only at this time) is a 250 gr Hornady FTX/Leverevolution/SST-ML bullet pushing 2200 fps. Remington is suppose to be offering a 250gr Core-Lokt sometime soon (or say they keep saying).. Others are reloading everything from 200 to 300 gr bullets
check out
www.450bushmaster.net - a forum entirely dedicated to the 450B.. If you reload this is the place to go as well.
Here is a couple pics of my rifle, ammo, and the accuracy i got at 75 yds
Brian