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?