I am interested in the .45 ACP Glock model 21 since I have found they offer it in the .50 version. At first I was looking to get a 1911 model, (the most familiar large caliber gun for me) but the price for what you get did not add up after looking into the Glock brand. I have watched a few videos on line and have a few questions to start. They have the .50 GI conversion that just slides on to either the model 20 (10-mm) or 21 (45-ACP). The Glock system does look sweet BTW. 1: Would the slide from another large frame fit the same lower assembly? I know that if I got the model 21 (45-ACP) I could also have a drop in slide and magazine in the .50 GI and even the .22 rim fire. That got me thinking. What about the interchangeability of the other large frame models? Glock 17 - 9-mm Glock 20 - 10-mm Glock 21 - 45-ACP Glock 22 - 40-SW Glock 31 - 357-SIG Glock 37 - 45-GAP Do I assume the 20 and 21 lower assemblies are the same as the .50 GI slide fits them both, and could I have an extra model 20 10-mm slide and mag to fit the model 21 frame? Will all, or any of those magazines and slides listed above fit the lower assembly of a model 20 or 21? I assume the 9-mm being a much lower powered round would need a less stout frame than a 10-mm, 40 SW and a 45 ACP, but do they all use the same lower assembly? That would be nice to have a small case with an extra set of slides and mags in .22 rim fire, 9-mm, 10-mm, 45-ACP, 40-SW and the 50-GI. Talk about a doomsday survival gun! Be able to shoot damn near any ammo you come across by just swapping out the slide and magazine!