Just bought an RRA 9mm Mid length upper. I plan on pairing this with a dedicated lower. Have been looking at the CMMG lowers but don't know that much about the 9mm ar rifle. My main concern is the mags and which fit without and mods to them. Has anyone ever used one of these and which mags do you use in them. Says they use uzi mags and colts. I really want to use the C- products or maybe a promag. Any thoughts, thanks.