It's 9x25 Dillon that the G20 can be converted to. It's a wildcat caliber that's not got many sources of ammo, (Double Tap being the only one I know of selling it) but it's a blast to shoot. Sort of a 9MM super magnum. Looks just like .357 Sig, but the case is longer.
9x25MM, 40 cal, .357 sig, and 10MM all are based on the exact same diameter & rim dimensions at the rear end of the case, and that's why a G20 can be converted to shoot any of them.
As for barrels:
http://www.barsto.com/category_main.cfm?ID=G20&cco=28
http://www.kkmprecision.com/custom_pistol_barrels/home.php
http://www.storm-lake.com/
Lone Wolf Distributors also has every kind of G20 barrel you could want. I don't think theirs are as good looking (as KKM especially), but they're considerably cheaper, and they're good enough that they ARE a pretty decent deal for the price.
The reason you cannot fire .45 in a G20 is that the breech face width in a 10MM isn't wide enough to fit the larger diameter of the .45 caliber cartridges. People have converted G21's to fire 10MM, but not the other way around. A G20 would need a different slide to shoot .45 caliber.
Almost forgot:
Be warned that Bar Sto's "Semi Drop-In" barrels often require final fitting to the individual gun, and unless you are able to do that yourself, the gunsmith work could cost you as much as the barrel did. LWD barrels are made to fit looser, like Glock factory barrels are, and they always drop in with no fitting needed.
As for putting G20 conversion barrels in a G21 - I've never tried, but if someone can verify that a G20 factory barrel will fit a G21, then I'm sure the conversion barrels would work too.
