It was posted in the Gen4 Problems thread that the recoil assembly is the only different part...but that's not so. From what I've seen, in addition to the recoil assembly there are the following differences: 1) The front of the slide is "bigger" in the "front end of the recoil spring area", necessitating the frame also have a roomier dust cover channel for the slide to fit...which is why a Gen4 slide can't be mated with an older Gen frame. 2) The trigger housing is different due to the MBS design. An older Gen trigger housing won't fit a Gen4 frame. 3) The trigger bar is different. I don't know (and would very much like to know) if it's ok/safe to put an older style trigger bar into a Gen4. If it can be done, are there any downsides to it? If not, any speculation as to why Glock did this? 4) Mag catch is different, not sure if they're interchangeable. (3) is my main reason for this post. I'm considering doing some work on my G17 Gen4 trigger, and wonder if its safe to use the trigger bar from a Gen3 G17?