I'm strictly using Pmags in my XCR. When I first got them I was struck by how similar they are to AUG mags--which is a good thing. In fact, I've been seriously considering picking up an AUG STANAG stock and using the Pmags in it as well.
I don't think the cap is really necessary to keep the lips from spreading. I think they're more for long term storage to minimize the chances of it happening over a year or more of disuse. On the other hand, Pmags are cheap and available enough that I don't much care. The one thing that the AUG mag has to it's advantage as opposed to the Pmag is the mag lips. They're about half again as thick on an AUG. But, at $14 as opposed to $30-$40, I'm willing to take that risk, heh.
I don't think the cap is really necessary to keep the lips from spreading. I think they're more for long term storage to minimize the chances of it happening over a year or more of disuse. On the other hand, Pmags are cheap and available enough that I don't much care. The one thing that the AUG mag has to it's advantage as opposed to the Pmag is the mag lips. They're about half again as thick on an AUG. But, at $14 as opposed to $30-$40, I'm willing to take that risk, heh.