I made a successful conversion from .45 - 10 mm with my 3rd Gen G21.
The components I used were:
- KKM .45 ACP : 10 mm Conversion Barrel
- Glock OEM 10 mm Magazines
- SS Guide Rod
- 22 lb. Recoil Spring
The guide rod and recoil spring were optional. Since the excess metal on the G20's slide is well ahead of the firing chamber, I figured it's a mass thingy. I thought the stiffer spring might compensate for the G21's somewhat lighter slide. At any rate, it works.
The conversion barrel's steeper feed ramp was a little picky about ammo. The square-nosed and polygonal-nosed ammos would occasionally fail to feed, whereas the bulbous-nosed ammos, like Double Tap's 135 grain Noslers feed just fine. The latter is my favorite 10 mm ammo!
My last three trips to the range were 100% free of any problems.
What a hoot, having my big, gentle kitten of a .45, sounding off like a lion!
The rig came in at about 1/2 the cost of a new G20, all fees considered. A used 10 mm would be hard to come by in my market--mine is the only 10 mm I've seen at my range! .45 ACP and .40 S&W rule in my neighborhood...
While it's fine for range play, I'd recommend a dedicated 10 mm for carry or hunting.
I think 10 mm is the bee's knees!
--Ray