I'm such a moron. I bought a used Glock 27 with night sites for $400 out the door about a week ago. I cleaned it (more on that later) and have had it in my holster until today when I went to the range. First mag was loaded with Hornady 155gr TAP ammo. I get through 2 rounds, slide locks back. Hmmm.....I though. Three more rounds, slide locks back. Tried the Tulammo I bought for next to nothing and same thing for mag after mag. Tried some Blazer brass ammo, same thing. Every two-four rounds the slide would lock back. Hit the slide lock and it would go back to firing until it happened again. I took it to the range counter and borrowed a Glock 23 mag and same issues. I was beginning to think I made a BIG mistake getting rid of my M&P9C.
Then I took it to the guy who owns the range (who is a Glock armorer) and he looks at it and says, your slide stop spring is bad. He took out the pins and said "who has been messing with this thing?" Sheepishly I admit I tried to take it apart and clean it (as in, removing the firing block/trigger assembly). He said "that is the problem". I apparently didn't put the pins back in correctly and the slide stop wasn't engaging. He fixed it, for free mind you, and I put 50 rounds down the pipe with no hiccups and pretty ridiculous accuracy.
It was a bumpy first 100 rounds, but the last 50 were a ton of fun and I love my baby Glock.
Moral: Don't mess with your gun unless you know what the heck you are doing!!!! God help me if I had gotten into a fight!