The 20 and 21 share the exact same frame. The 20 can use caliber conversion barrels in .357 Sig, .40 S&W and 9 X 25. The 10mm mags work fine. Lone Wolf makes a .45 ACP slide for the G20 frame. You can get conversion barrels in .400 CorBon, .40 Super and .460 Rowland. You do need .45 ACP mags for the .45 based cartridges. The .460 is specifically NOT recommended by Glock. That is a VERY high energy round. It is probably best left to steel framed 1911's.
I have a G-20 and Lone Wolf barrels in .357 Sig, .40 S&W and a conventionally rifled 10mm. All work well.
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