I have had a problem the last two times I have went to the range with my 21. It will about fifty percent of the time fire then the slide locks to the rear as if the mag is empty. It will do it whether it is full or close to empty with no differance and completely random . I may get a couple shots off and then every one will lock back. The mag appears to be ok, I can't see any damage to the feed lips or see if they bent in any way. I only had one mag with me today, but last time I had three and cannot recall if it was all of them or just this one. What should I look for?

Is the slide stop locking back the slide, or is the top round in the mag not stripping off and jamming the slide back?

Check your slide stop.  Make sure it still has some spring tension to keep it from locking the slide.  It that's the culprit, it's a $5 part and an easy replacement.

Check your mag.  Make sure the top round strips off easily.  Again - simple fix if the mag is the problem.  Toss it and buy more.  Most places have them for about $20 each.

If the slide is in fact locking back on the slide stop, check the slide stop for correct installation.  If the pin was removed, the slide stop spring may not be positioned correctly.  

Quoted: Is the slide stop locking back the slide, or is the top round in the mag not stripping off and jamming the slide back? Check your slide stop.  Make sure it still has some spring tension to keep it from locking the slide.  It that's the culprit, it's a $5 part and an easy replacement. Check your mag.  Make sure the top round strips off easily.  Again - simple fix if the mag is the problem.  Toss it and buy more.  Most places have them for about $20 each.

This is my guess.

Do you have after market parts? You always have to ask this when a Glock has a problem.

You know as stupid as it sounds I did not check to see if the slide lock was in fact locking it although it was not from it stripping a round and jamming forward, the rounds were still in the top of the mag in postion. I would rack the slide and it would 30% of the time chamber the next round, sometimes it would take a second time to chamber because the first time it would just go to lock, and yet again sometimes I would have to drop the mag to get it to work upon reinserting the mag. It does have a extended mag release on it and the spring pressure seems light, but I do not recall the pressure from when installed.
   Now before these two times it worked flawlessly for a couple thousand rounds, the first time I had to use range ammo and was a cheaper brand and at first thought it may of been an ammo prob.

As the others have said, check for proper installation of the slide stop lever. Also inspect the lever itself to see that the spring is in the proper position.

You may also want to check your grip. When shooting from a thumbs forward grip it is possible for the strong hand thumb or weak hand palm/thumb are to contact the slide stop lever (especially under recoil). This can cause either the slide to not lock back on empty or in some cases the slide will lock back prematurely. This problem is usually made much worse by the use of extended slide stop levers.

Best of luck in your diagnosis!

Thanks for the input. I was typing this as I had the gun apart and when I put it back together I see that the slide release has no spring tension, at least I know the culprit. Thanka again for the spot on diagnosis.

Quoted: Thanks for the input. I was typing this as I had the gun apart and when I put it back together I see that the slide release has no spring tension, at least I know the culprit. Thanka again for the spot on diagnosis.

Get rid of the after market part. It is a slide stop not a slide release.

Quoted: Quoted: Thanks for the input. I was typing this as I had the gun apart and when I put it back together I see that the slide release has no spring tension, at least I know the culprit. Thanka again for the spot on diagnosis. Get rid of the after market part. It is a slide stop not a slide release.

True.