It's something about the way the very top of the mag is contacting something. If I wiggle it past the first few millimeters, it will drop the rest of the way on it's own. Just the follower pushing against a closed bolt is enough to make it drop. Open bolt, nothing drops free, no matter what, until it's past those first few millimeters. I only have one KAC mag, I guess it's possible I have a dud, but at $100 a piece I'm not going to buy more to find out. I picked up two C-Products magazines as well, neither of them drop free and one doesn't even fit. The only type of mag I have a significant number of is the PMAGs.
I started this thread back when I first got the rifle:
http://www.snipershide.com/shooting...rifles/99899-sr-25-type-magazine-options.html
I've sanded the PMAGs, and that's been the only thing that's provided anything like success, although it's still hit or miss whether they fall out or stick.