Sorry if this has been asked a hundred times before, i’ve tried to do research but can’t seem to get a consistent answer…I’m looking to do an AR9 build with CMMG RDB system (already have the BCG & Barrel kit), if I run CMMG’s MkGs for glock mags, do i need to run their upper with it, or can I use any AR-15 upper? I’ve read you can’t use a typical AR9 upper due to ejection port problems, but i haven’t been able to tell if you HAVE to use a Mil-Spec AR-15 upper or if you could use an aftermarket.
Again sorry if this gets asked a lot, i just don’t want to buy the wrong parts and have my firearm malfunction…

Correct.  You need the full length port for the RDB to eject reliably.  Billet or forged shouldn't matter.

@amphibian

Some links from my site that you may find helpful:
CMMG RDB Ejector spring issues
My OLD CMMG RDB setup - tuning info