I can probably answer your questions comlpro.....what do you need/want to know?
By the way Izzy, your friend is mistaken regarding the .45GAP round, it is about as powerful as .45acp, but the case is shorter thereby reducing the frame. It is "small hands friendly". The glock 21, in .45 caliber is a large frame pistol. The glock 39 is a medium frame.
Also you can swap barrels/calibers with a couple of things to remember....
1)medium frames to medium frames
2)always buy the largest calibre in that frame, then buy your smaller conversion barrel...if you don't the barrel may not fit into the hole in the front of the slide for the barrel. You in essence are getting a bull barrel in the smaller caliber.
I have done this for my glock 22....
Have a conversion barrel for .357sig, which is a .40 calibre bullet necked down to .357.
Both can/do use the clip for .40
I also have a 9mm barrel for it. When using the 9mm barrel you MUST use 9mm clips, kinda makes sense huh
Like I said any questions I'll do my best to help ya.

-Bill Liptak
Rocking the combat tupperware