Hey! I registered on this forum for this thread. I have a 3rd Gen. (3 pins) G17. I am experiencing the same problem the first poster has with his G19. The magazine does not completely fill the cavity designed to hold it.
I do not have any feeding issues, but the small gap does become annoying especially with the extended 33 round mags. I have remedied this issue by placing a few small pieces of electrical tap on the front and side of my G17 mags and building it up to fill the void. I am not happy with the way it looks but the rattle was driving me insane. I tried to make the pieces of tape as small as I possibly could. I have even thought about placing the tape inside the grip instead of the mags.
I will have to go and see if I can find any of the older mags. My G36 does not have this problem.
I just wanted to ad my two cents!