Quoted:
Last night I had multiple issues with a FDE 20 round Pmag jamming when I only had a couple rounds in the mag.  I took the mag apart and cleaned it same result. If I had 5 or more rounds loaded it would chamber fine.  I few times while shooting with a full 20 when it got low it would jam on feeding. I switched over to a brand new 30 round Pmag and didn't have an issue wither it was 2 rounds or 30 rounds.  I tested my other 2 20 rounders at home with magpul dummy rounds and got the same exact results. 2 or 4 rounds in the mag rack the CH on a closed bolt and FTF. use the bolt release and FTF. load 3 or 5 rounds and would chamber perfectly. eject that 3rd or 5th round and chamber the next and FTF BUT  if I was firing MOST of the time it would function fine. I think 3 or 4 times it would FTF on that second to last round.

I also tested 10 of my 30 round pmags with the dummy rounds and every one of them fed fine. I tested all the magazines in a second AR and the 20 rounders jammed every time.

I am thinking maybe the springs might be weak I don't know really. Hoping to get some feed back from magpul on this but below is a picture to help illustrate what is happening. the bolt releases and the bullet jams there as it is seen in the picture and wont chamber usually resulting in a damaged round. doesn't matter if it was brass cased or steel cased or a plastic dummy round

http://img.photobucket.com/albums/v508/x2122/2012-10-02_21-50-33_334.jpg