Guys, what the hell am I missing? I have a pretty early gen 4 G17. I have shot thousands of rounds through it with zero problems. Only aftermarket parts are the extended slide lock that has been on since day 1, Ghost connector and extended mag release. It also has an RMR milled in it by Marc H. I was running a pistol 2 class a few weeks ago with a DIRTY gun. 1/3rd the way through the class I started getting premature slide lock backs. Here are all the variables. -Slide ultimately locked back with every one of my 9 G17 mags and 1 G19 mag. -Slide locked back with different number of bullets left every time but always *seemed to be towards the end of the mag (maybe last 7-1 rounds would be left in the mag when it would lock back.) -Even though my grip has never really changed, I diligently made sure my thumbs were not riding the slide stop -In the thousands of rounds shot, this is the first time I have ever tried a different kind of ammo when this problem started. It was PMC 115 Grain FMJ from SG Ammo. I ALWAYS use federal maroon box in the past. Could this possibly be an ammo problem? It is the ONLY thing I could think of that changed. -Slide stop spring IS under the top pin. -slide stop snaps back down nice and fast when lifted up and released (does not seem faulty in any way) So after the class I took it home and cleaned the hell out of it and just hoped it was due to the gun being hella dirty. Well I took it out again and sure as **** it is still doing it, with all 4 of the mags I brought out. I really dont think it is a faulty mag since it did it with all of them, it was a clean gun, it is put together correct, the lever and spring seem more then fine, im not riding it. literally the only thing I can think of is the different ammo. Like an idiot I didnt bring any federal with me. Only the PMC. What am I missing guys? what can I possibly be overlooking that is causing this problem?