I just completed my 5.7 build and realized that I simply messed up and didn't read the fine print on the CMMG site that their 5.7 conversion mags only work with their proprietary upper receiver which they don't sell stripped (of course) and only as complete. So basically the mags don't lock in with the bolt closed, lock in with it open but soon as its released it hits the mag before catching the bullet to feed into the chamber. If I hold the mag to the standard position to where it should be it feeds and ejects no problem so I'm assuming CMMG just decided to engineer the mags to need you to spend 1k on their uppers that only have some kind of lip that stops the mags from going up into the mag well further than they should lol. I was going to take the gun to my gunsmith and see if there was an was solution that could be fabricated so that I can utilize the CMMG mags that I already have, but I was also wondering if there are any other 5.7 AR mags or drum available that would also work?

Sorry, couldn't resist

I am not familiar with the system you have, but, if the mags are sitting too high perhaps you can modify the magazine or mag catch enough to compensate?

A few years ago when CMMG came out with the 5.7 RDB setup I assembled an AR with their 8” barrel/bolt combo.

I used a Mega lower, Aero upper and BCM rail and it went together like any other AR build. I’m using the CMMG

mags that fit a standard AR lower and I have not had any problems.
Attached File

Ah I see exactly what happened you have the first gen mags... I have the second gen mags so they obviously made this change with the Gen 2 mags.

UPDATE..  i tried the CMMG mags on a couple of my other ARs and the mags locked in perfectly on all of them but then I tried my other AR mags on the AR57 and they did the same thing the CMMG mags did. So the problem seems to me be with the mag catch on the gun itself being too recessed to catch the mag which I can actually see.

That was an old photo.  I have since SBR’d this gun and also have some gen 2 mags.  I went back and read CMMG’s description and
it says to use their upper or a mil spec upper. Not sure why you are having problems.
Attached File

moe3161 just saw your update hopefully a new mag catch will fix your problem.

yea thats the part that threw me off was because they said designed to work with "CMMG 5.7x28 Radial Delayed Blowback™ Uppers and Mk4/Mil-Spec AR15 lower receivers". But yes I took it back to the shop and they just had to make an minor adjustment and it should be good to go now.