I am a Bushmaster fan and have a few of their ARs. If you are searching the market for a 9mm AR I would highly recommend checking out Spikes Tactical. If you are set on Bushmaster by all means. I'm sure you won't be disappointed.
Check out RRA They have detacated 9mm lowers, no mag blocks to mess with. I have had Colt, it was very good, but it has a block. I have
a RRA 9mm M4 that I made a SBR out of. Great/fun Good luck