I want to post this here for both Magpul and any other frustrated PMAG owners, hopefully it helps someone else!
I bought a couple 20rd and 30rd PMAGs a while back, and I have been cursing ever since because I've had several problems with them. It's not with a single rifle either, these same things happen with two separate Armalite rifles (16" mid & 20" rifle).
The problems are:
1. Fail to hold open on last round. Holds open if I pull the bolt back manually with the charging handle. This happens on both 20rd & 30rd PMAGs.
2. Nearly impossible to use the bolt release, have to press hard...usually have to hit it with my hand to release the bolt after loading a new mag. This happens on both 20rd & 30rd PMAGs, worse on the 20rd.
3. Doesn't drop free from mag well. Only happens with 20rd PMAGs.
4. Really hard to insert fully loaded mag on a closed bolt, have to smack it with my palm to get the mag to latch in. I don't care about this too much, I only load 5-10 at a time usually.
I have seen these problems posted by frustrated users on several forums, but I haven't seen a reasonable fix posted yet. So today I did some thinking and checking, and the answer became pretty obvious. The short version of the story is that there are a couple tabs at the top of the back of the magazine that you have to trim off because the interfere with the bolt catch.
This should fix issues #1 and 2 from the list above.
After this fix the bolt release is now easy to press, and it appears to hold open without any problem.
I have not been to the range yet, I will go tomorrow and report back the results in this thread.
Picture 1: These tabs need to be trimmed off.
http://img193.imageshack.us/img193/1837/pic1ke0.jpg
Pictures 2&3: Tab nearest the ejection port hits on bolt catch and makes it either stick down (causing failure to hold open on last round) or stick in the up position (causing difficulty to press bolt release).
http://img230.imageshack.us/img230/3259/pic2l.jpg
http://img403.imageshack.us/img403/7979/pic3qv.jpg
Picture 4: Trimming tab with a saw, then clean up the cuts with a knife.
http://img194.imageshack.us/img194/8586/pic40.jpg
Pictures 5&6: Before (left) and after (right) the surgery.
http://img190.imageshack.us/img190/2060/pic5n0.jpg
http://img256.imageshack.us/img256/3629/pic7i.jpg
Picture 7: Comparison of various 20rd mags (left to right): TangoDown ARC, USGI aluminum, PMAG post-surgery, PMAG original
http://img405.imageshack.us/img405/2585/pic6bdn.jpg
I hope this post has been useful to some other people here and to the Magpul team!