I recently dipped my toe in the 450BM waters with one of the Bushmaster uppers from Midway. Mine has a 20" barrel and came with a magazine.
I loaded up some rounds and headed to the range. The lower has a stock buffer and spring. Ejection was very spotty and I gave up trying to load more than one round at a time into the magazine. The brass would occasionally eject between 1 o'clock and 2 o'clock (flying around ten feet away) but more often would either barely make it out of the ejection port or stay in the gun, laying on top of the empty magazine.
Since this is a Bushmaster upper chambered in 450 Bushmaster it does have the ejection port opened up already. Basically the portion rear of the dust cover detent measures around 0.51" (doesn't appear to have been touched) while the area forward of the detent is a uniform 0.55" or so. Is this sufficient or does it possibly need to be opened up more? I would think that Bushmaster would open it up enough from the factory but perhaps not.
I don't suspect the loads but just in case:
Starline Brass
250gr GTX bullets
2.25" COAL
CCI SRP
LIL GUN (four rounds each with a charge range of 36.4 to 38.2 in 0.2gr increments)
0.478 taper crimp
The problems occurred regardless of the powder charge.