Which way conversion Glock 20/21?

Well if you're using a "complete" upper, slide and barrel, it can go either way. The lowers are identical.

However if you have a 21 you can just get a 10mm conversion barrel and 10mm mags fior it and don't have to change the slide. Be aware though that a 20 slide is heavier and the lighter 21 slide will reach higher velocities and may cause battering to the frame, so you may want to get a heavier recoil spring or a buffer to try and compensate for the lighter slide weight.

This doesn't work the other way because the breechface on a 20 slide is thinner and the .45 cases will not fit into it.

Also be aware that the difference in breechface width could cause feed and extraction problems so I wouldn't use a conversion for actual self-defense use.


If you are planning to use the pistol for defensive purposes then a complete upper is the way to go.

Weevil, is correct. I used to own both a 3rd gen G20 and a 3rd gen G21 (yep, still kicking myself for selling them off) and the frames and all the parts in the frames (the ejector, the locking block, the trigger bar assembly, etc) are all identical between the two. The same is true between a G29 and a G30 frame.

Thanks guys; I was told by the folks in Smyrna you couldn't do a 20 upper on a 21 lower; didn't sound right to me. Thanks, TF

Actually, this kinda makes sense. First off, you would think that they are both identical in every way, as did I. However, owning both a G21 and G20, it seems that Smyrna might be correct. The reason they would say that the complete G21 upper would work on the G20 lower, but not vice versa, is probably due to the very weak plastic pieces on the frame that the recoil guide rod pushes against.

I've used .40 Super and .460 Rowland, utilizing the extremely stiff EFK Firedragon RSA on the G21 frame, and it broke that weak plastic and the slidelock lever. Looking at those pieces in contrast/comparison to the G20, the G20 appears to be THICKER at those spots. As with the thicker frame on the G20, this is more than likely due to the engineering/building for the much higher pressures of the 10mm (esp. Full House loads).

So, perhaps the stronger built G20 can handle the "typical", weaker .45acp wear and tear, but the same might not be possible the other way around. Now, I am sure you can do it, for a while... But, for how long? Mine lasted about 300 or so rounds of .40 Super, .460 Rowland, .45acp.

AS somebody else correctly posted, the Glock 21 and G20 lowers are identical. If somebody at Glock/Smyrna told anybody different, then I'd say the caller got the wrong person on the phone. My experience with a G21 10mm conversion barrel was dismal. I wouldn't recommend that conversion to anybody.

Just be aware that an authoritative-sounding looooong post just might be a post fraught with fallacious information.

So, since this point has only been made two or three times, let's repeat it again. G21 and G20 frames are identical!!!!!

Is the ejector different? That's easy enough to replace, anyway.

If my math is right, you only save $200-250 buying two complete uppers (and that's if you buy a used, complete upper at the market price). And you still have to store two sets of mags and ammo (the actual firearm is the small bit). Unless I walked right into a complete upper for WELL under $300.00, I wouldn't go that route, unless you live somewhere that the number of firearms purchases is limited, maybe.

If you buy two complete guns, you could get an SF and an SF ambi, perhaps, just to have more options.

Is the ejector different? That's easy enough to replace, anyway.

Click to expand...

See large letters in post #7!!!

Well, that's interesting. The 40 and 9mm use a different ejector, and those calibers are much closer in diameter.