I just bought a 223, looking at mag options. I like the AR mag idea. GI mags are everywhere around here, used and new. Go in anywhere and ask for a galil mag, or and 223 AK for that matter, and you get a blank stare.
The MSA adapter does look nice, I've done a bit of fab work but not on guns though. I'm a bit apprehensive about taking a grinder to new parts. A little is Ok but how much is really necessary to install this thing? I do have a dimpled receiver.