I recently purchased a used glock 18. I took it to the range. I started with two magazines of 15 in semi mode just to warm it up and then followed with a magazine of 30 (extended). I had no problems with the jamming so far. the gun was tip top. I later switch to the standard magazine filled it with 15 rounds in auto mode. Then the jamming started, after a three burst shoot the gun will jam. An unfired round will try to get into the chamber but the slide wouldn't close entirely so i had to release the magazine and cock the gun again. I was firing the FMJ winchester 115gr. and then i switched to magtech 124 gr FMJ. but it would still jam after a three burst shoot. it's so weird because in the first round of 30 the gun didn't jam. I was wondering if any of the glockers on the forum know anything about this. the extended magazine was brand new. my gun is compesated, but it's a custom made where they drill five holes on top of the slide. i was thinking if springs could be the problem, or if the barrel is over compensated. the weird thing is that it didn't have any problems in the beginning. if anyone could have some suggestion, i'll be very glad to take them.