9mm AR vs .300 Blackout

[fledge]

you guys have it all wrong. the issue with 9mm ARs is they are making the LOWER fit the 9mm and not the upper. I'm waiting for someone to make a proprietary 9mm upper that works on a standard AR lower and has mags that are the shape of a 30rd 5.56 mag. L

I'd get behind that. And the magazine as an insert that goes in a standard pmag. This can't be that hard.

CMT's upper will be an original design around the 9mm. If only it worked with an SSA lower...