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…