It's have a dumb question.... I have a 10mm barrel can this be dropped into the gen1 g21 and function properly? Would the mags have to be altered as well?
No. The slide thickness is different between the 21 and the 20. The 20 slide is thicker and the barrel for the 20 is thicker than that of the 21.
KKM makes a .45ACP-10mm conversion barrel for the G21. It requires use of G20 magazines, and they recommend making a specific modification to the G21 extractor so that it will function best with both the stock .45ACP barrel and their conversion barrel, but you have to call them to get information on that extractor modification, I've never seen it discussed openly anywhere. If you were going to shoot decent strength 10mm loads, I'd get an aftermarket guide rod with a heavier spring, at least 20lbs. The factory recoil spring weight is 17lbs. The G20 uses a heavier slide which keeps the slide in battery longer while using the same strength recoil spring as the G21, so I'd use a heavier recoil spring for the conversion unless you're shooting light 10mm loads.
