Lone Wolf also makes a buttload of conversion barrels for Glocks...
I have their 9" barrel on my G20-SF
With a G21 swap, its the same frame as the G20, slide hole is slightly bigger, of course...
and get a 10mm/G20 Extractor & Recoil Spring, as that solves the extraction issues...
You can even use a Standard Glock 20 barrel, but where's the fun in that??
Oh, you can "Get Away With" not switching the Extractor & Recoil Spring...
however, switching them makes it work PROPERLY....as opposed to battering your slide stops,
and occasionally not pulling the case from the new barrel or stovepiping it...