I have 3 P99 mags all of which physically look identical. One came with the gun and 2 I bought from a dealer online that are stamped LE/Military use (pre-bans).
Well, when I put each empty mag in, and pull the slide back 2 of the 3 work fine. The 3rd (one of the 2 I bought online) is now not locking the slide all the time, and when it does "lock" it's more of a partial lock.. a slight nudge to the slide and it'll come off the slide lock.
Now, to resolve this problem I tried putting my hand on the bottom of the mag to hold it in securely. This seemed to fix it. The slide lock would fully engage.
Now, here's the stinker... I did not drop the gun or mag at any time, they all LOOK exactly the same. I examined them all thoroughly. I can't think of what has happened but something obviously did. It happened after my recent trip to the range. Got home and realized the slide wasn't locking properly on the one mag. Yet it works FINE on the other 2 mags which leads me to believe something is wrong with that mag...
but WHAT?!