Inserted by MP:
Siimple explanation and Simple fix of what is the problem is. It took me all of about 10 seconds
What the problem is:
The lip on the un-fixed P-mag pushes the mag release/Bolt catch bar up just enough to catch the bolt group, but not enough to stop it. This will severly damage your bolt group over time. Also if you rest your gun on the mag when you shoot(by the way don't do this) it will shove most mags including FN factory ones.
1.) It is real simple. I used a dremmel and just shaved off about 1/16 of an inch(guessing). SEE PHOTO #1
2.) I took my lower off my SCAR and tested them( You have to push the magazine follower down in order to test).
3.) if the bar lays flat, you have fixed it.(see Picture #4)
I also tested all my other "metal" magazines and had to shave a few of them(including my FN mags)
FIXED MAG ON LEFT
FRONT VIEW OF THE RELEASE BAR HITTING THE UNFIXED MAG
WHAT THE UN-FIXED MAG DOES
WHAT THE CORRECTED MAG SHOULD LOOK LIKE(SEE HOW THE BAR LAYS FLAT AND DOESN'T INTERFERE WITH THE BOLT GROUP)
Over in the "wear and tear" thread we were coming to the preliminary cinclusion that PMAGs are forcing teh bolt catch up and may be forcing the bolt carrier up a bit, causing the bolt locking lugs to suffer some wear with impact to the chamber locking area. I have noticed previously that unmodified PMAGs do not allow the bolt catch to go fully down when the magazine is seated and loaded. I noticed the problem at about 500 rounds and modified all my PMAGs as described below, Since then I have not noticed any additonal wear on the bolt catch. I wasn't aware of potential bolt lug damage before the "wear and tear" thread, so I don't know if the bolt lug peening has ceased, but on mine it is very minor (I can only detect it if I know where t look and shine a light to see the shiny bits), and the gun has had 500 rounds since them. I believe this mod resolved the problem, but we'll see. Hope these pics help those who, like me, just LOVE PMAGs.
This pic shows the OEM magazine seated. Note the proper fully down position of the bolt catch:
This next picture shows an unmodified PMAG seated (I removed the internals for clarity). Note how far the catch is forced up. It's not enough to hold the bolt open, just enough for the bolt carrier to rub along the top of it:
Here's a pic of the wear on the bolt catch caused by this:
Here's the OEM mag (left) compared to the PMAG (right). Note the different rear opening. The bolt catch on the AR platform is directly in the center of the mag, so the shape of the notch doesn't matter on the AR, but on the SCAR, the bolt catch is offset slightly to the left.
Here's the area on the PMAG causing the problem:
The fix is relatively simple. Use a round file to remove a little bit of material from the spot highlighted above so that it looks like this:
A before and after comparison. You don't need to remove a lot of material. Keep the trigger housing (the lower) handy and as you remove material, seat the mag and see if its forcing the bolt catch up. When it no longer does, STOP, you're done.
Lastly, here's the modifed mag seated... no longer forcing the bolt catch up:
Hope that helps!