I recently got a good deal on a Gen 2 G23, but would prefer to have a spare G19. So I'm planning to either trade the G23 for a G19 or trade the complete upper (plus mags and ejector) for a G19 upper and turn the G23 into a G19. Before I do that I just wanted to be sure I wouldn't have any problems running a G19 slide (either Gen 2 or Gen 3) on my G23 Gen 2 frame. I noticed that the rear frame rails on my G23 are significantly longer than on 9mm Glocks. Do the longer frame rails cause problems in 9mm? I was always under the impression that once 9mm Glocks switched to 3 pin frames the frames were identical across 9mm/.40/.357sig, right? If so, why did Glock switch to shorter frame rails on the .40s?