Drop-In Builds

Which parts swap between a Glock 20 and a Glock 21?

The Glock 20 (10mm Auto) and Glock 21 (.45 ACP) are built on the same large frame. The caliber lives in the slide, barrel and magazine, so those are the parts that have to match each other.

The Short Answer

  • The G20 and G21 share the frame, trigger parts and slide parts kit. The slide decides the caliber.
  • A 10mm slide needs a G20 barrel and G20 magazines; a .45 ACP slide needs a G21 barrel and G21 magazines. The mags fit the same frame but feed different cartridges.
  • A G20 can shoot .40 S&W with a 10mm-to-.40 conversion barrel, using the same G20 magazines.
  • A Gen3 slide fits a Gen4 frame with the Gen3 single-spring recoil assembly. A Gen4 slide doesn't fit a Gen3 frame without cutting the frame.
  • Threaded barrels differ by caliber: 9/16x24 on 10mm and .578x28 on .45 ACP. The muzzle device has to match.
Check Your Own Glock 20 / 21 Build

Frame and Slide

Every large frame we list, checked against every G20 and G21 slide.

Glock G20/G21 Gen3 Frame (OEM, stripped)

  • Fits Glock G20 Gen3 Slide (OEM, stripped) · Brownells RMR Cut Slide for Glock 20 Gen3 · Zaffiri Precision ZPS.2 RMR Cut Slide, Glock 20 Gen3 · Glock G21 Gen3 Slide (OEM, stripped)
  • Won't fit Glock G20 Gen4 Slide (OEM, stripped) · Glock G20 Gen4 MOS Slide (OEM, stripped) · Glock G21 Gen4 Slide (OEM, stripped) · Glock G21 Gen4 MOS Slide (OEM, stripped) A Gen4 slide doesn't fit a Gen3 frame without cutting the frame for the dual recoil spring.

Glock G20SF/G21SF Gen3 Short Frame (OEM, stripped)

  • Fits Glock G20 Gen3 Slide (OEM, stripped) · Brownells RMR Cut Slide for Glock 20 Gen3 · Zaffiri Precision ZPS.2 RMR Cut Slide, Glock 20 Gen3 · Glock G21 Gen3 Slide (OEM, stripped)
  • Won't fit Glock G20 Gen4 Slide (OEM, stripped) · Glock G20 Gen4 MOS Slide (OEM, stripped) · Glock G21 Gen4 Slide (OEM, stripped) · Glock G21 Gen4 MOS Slide (OEM, stripped) A Gen4 slide doesn't fit a Gen3 frame without cutting the frame for the dual recoil spring.

Glock G20/G21 Gen4 Frame (OEM, stripped)

  • Fits Glock G20 Gen4 Slide (OEM, stripped) · Glock G20 Gen4 MOS Slide (OEM, stripped) · Glock G21 Gen4 Slide (OEM, stripped) · Glock G21 Gen4 MOS Slide (OEM, stripped)
  • Fits, with a note Glock G20 Gen3 Slide (OEM, stripped) · Brownells RMR Cut Slide for Glock 20 Gen3 · Zaffiri Precision ZPS.2 RMR Cut Slide, Glock 20 Gen3 · Glock G21 Gen3 Slide (OEM, stripped) A Gen3 slide fits a Gen4 frame and uses the Gen3 single-spring recoil assembly.

Lone Wolf TWF Large Frame, Textured (stripped)

  • Fits Glock G20 Gen3 Slide (OEM, stripped) · Brownells RMR Cut Slide for Glock 20 Gen3 · Zaffiri Precision ZPS.2 RMR Cut Slide, Glock 20 Gen3 · Glock G21 Gen3 Slide (OEM, stripped)
  • Won't fit Glock G20 Gen4 Slide (OEM, stripped) · Glock G20 Gen4 MOS Slide (OEM, stripped) · Glock G21 Gen4 Slide (OEM, stripped) · Glock G21 Gen4 MOS Slide (OEM, stripped) A Gen4 slide doesn't fit a Gen3 frame without cutting the frame for the dual recoil spring.

Slide and Barrel

The barrel has to match the slide's caliber.

Glock G20 Gen3 Slide (OEM, stripped)

  • Fits Glock G20 Barrel (OEM) · Lone Wolf LWD G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24 · Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel
  • Won't fit Glock G21 Barrel (OEM) · Lone Wolf AlphaWolf G21 Barrel, .45 ACP · Lone Wolf LWD G21 Barrel, Threaded .578x28 · Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28 This barrel is made for a G21 slide, not a 10mm Auto slide.

Brownells RMR Cut Slide for Glock 20 Gen3

  • Fits Glock G20 Barrel (OEM) · Lone Wolf LWD G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24 · Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel
  • Won't fit Glock G21 Barrel (OEM) · Lone Wolf AlphaWolf G21 Barrel, .45 ACP · Lone Wolf LWD G21 Barrel, Threaded .578x28 · Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28 This barrel is made for a G21 slide, not a 10mm Auto slide.

Zaffiri Precision ZPS.2 RMR Cut Slide, Glock 20 Gen3

  • Fits Glock G20 Barrel (OEM) · Lone Wolf LWD G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24 · Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel
  • Won't fit Glock G21 Barrel (OEM) · Lone Wolf AlphaWolf G21 Barrel, .45 ACP · Lone Wolf LWD G21 Barrel, Threaded .578x28 · Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28 This barrel is made for a G21 slide, not a 10mm Auto slide.

Glock G20 Gen4 Slide (OEM, stripped)

  • Fits Glock G20 Barrel (OEM) · Lone Wolf LWD G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24 · Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel
  • Won't fit Glock G21 Barrel (OEM) · Lone Wolf AlphaWolf G21 Barrel, .45 ACP · Lone Wolf LWD G21 Barrel, Threaded .578x28 · Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28 This barrel is made for a G21 slide, not a 10mm Auto slide.

Glock G20 Gen4 MOS Slide (OEM, stripped)

  • Fits Glock G20 Barrel (OEM) · Lone Wolf LWD G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24 · Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel
  • Won't fit Glock G21 Barrel (OEM) · Lone Wolf AlphaWolf G21 Barrel, .45 ACP · Lone Wolf LWD G21 Barrel, Threaded .578x28 · Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28 This barrel is made for a G21 slide, not a 10mm Auto slide.

Glock G21 Gen3 Slide (OEM, stripped)

  • Fits Glock G21 Barrel (OEM) · Lone Wolf AlphaWolf G21 Barrel, .45 ACP · Lone Wolf LWD G21 Barrel, Threaded .578x28 · Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28
  • Won't fit Glock G20 Barrel (OEM) · Lone Wolf LWD G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24 · Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel This barrel is made for a G20 slide, not a .45 ACP slide.

Glock G21 Gen4 Slide (OEM, stripped)

  • Fits Glock G21 Barrel (OEM) · Lone Wolf AlphaWolf G21 Barrel, .45 ACP · Lone Wolf LWD G21 Barrel, Threaded .578x28 · Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28
  • Won't fit Glock G20 Barrel (OEM) · Lone Wolf LWD G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24 · Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel This barrel is made for a G20 slide, not a .45 ACP slide.

Glock G21 Gen4 MOS Slide (OEM, stripped)

  • Fits Glock G21 Barrel (OEM) · Lone Wolf AlphaWolf G21 Barrel, .45 ACP · Lone Wolf LWD G21 Barrel, Threaded .578x28 · Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28
  • Won't fit Glock G20 Barrel (OEM) · Lone Wolf LWD G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, 10mm · Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24 · Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel This barrel is made for a G20 slide, not a .45 ACP slide.

Slide and Magazine

So does the magazine.

Glock G20 Gen3 Slide (OEM, stripped)

  • Fits Glock G20 15-Round Magazine (OEM) · Glock G20 10-Round Magazine (OEM) · ETS G20 15-Round Magazine · ETS G20 20-Round Magazine
  • Won't fit Glock G21 13-Round Magazine (OEM) · Glock G21 10-Round Magazine (OEM) · ETS G21 13-Round Magazine · ETS G21 18-Round Magazine A 10mm Auto slide needs G20 magazines; this is a .45 ACP magazine. They fit the same frame but feed different cartridges.

Brownells RMR Cut Slide for Glock 20 Gen3

  • Fits Glock G20 15-Round Magazine (OEM) · Glock G20 10-Round Magazine (OEM) · ETS G20 15-Round Magazine · ETS G20 20-Round Magazine
  • Won't fit Glock G21 13-Round Magazine (OEM) · Glock G21 10-Round Magazine (OEM) · ETS G21 13-Round Magazine · ETS G21 18-Round Magazine A 10mm Auto slide needs G20 magazines; this is a .45 ACP magazine. They fit the same frame but feed different cartridges.

Zaffiri Precision ZPS.2 RMR Cut Slide, Glock 20 Gen3

  • Fits Glock G20 15-Round Magazine (OEM) · Glock G20 10-Round Magazine (OEM) · ETS G20 15-Round Magazine · ETS G20 20-Round Magazine
  • Won't fit Glock G21 13-Round Magazine (OEM) · Glock G21 10-Round Magazine (OEM) · ETS G21 13-Round Magazine · ETS G21 18-Round Magazine A 10mm Auto slide needs G20 magazines; this is a .45 ACP magazine. They fit the same frame but feed different cartridges.

Glock G20 Gen4 Slide (OEM, stripped)

  • Fits Glock G20 15-Round Magazine (OEM) · Glock G20 10-Round Magazine (OEM) · ETS G20 15-Round Magazine · ETS G20 20-Round Magazine
  • Won't fit Glock G21 13-Round Magazine (OEM) · Glock G21 10-Round Magazine (OEM) · ETS G21 13-Round Magazine · ETS G21 18-Round Magazine A 10mm Auto slide needs G20 magazines; this is a .45 ACP magazine. They fit the same frame but feed different cartridges.

Glock G20 Gen4 MOS Slide (OEM, stripped)

  • Fits Glock G20 15-Round Magazine (OEM) · Glock G20 10-Round Magazine (OEM) · ETS G20 15-Round Magazine · ETS G20 20-Round Magazine
  • Won't fit Glock G21 13-Round Magazine (OEM) · Glock G21 10-Round Magazine (OEM) · ETS G21 13-Round Magazine · ETS G21 18-Round Magazine A 10mm Auto slide needs G20 magazines; this is a .45 ACP magazine. They fit the same frame but feed different cartridges.

Glock G21 Gen3 Slide (OEM, stripped)

  • Fits Glock G21 13-Round Magazine (OEM) · Glock G21 10-Round Magazine (OEM) · ETS G21 13-Round Magazine · ETS G21 18-Round Magazine
  • Won't fit Glock G20 15-Round Magazine (OEM) · Glock G20 10-Round Magazine (OEM) · ETS G20 15-Round Magazine · ETS G20 20-Round Magazine A .45 ACP slide needs G21 magazines; this is a 10mm Auto magazine. They fit the same frame but feed different cartridges.

Glock G21 Gen4 Slide (OEM, stripped)

  • Fits Glock G21 13-Round Magazine (OEM) · Glock G21 10-Round Magazine (OEM) · ETS G21 13-Round Magazine · ETS G21 18-Round Magazine
  • Won't fit Glock G20 15-Round Magazine (OEM) · Glock G20 10-Round Magazine (OEM) · ETS G20 15-Round Magazine · ETS G20 20-Round Magazine A .45 ACP slide needs G21 magazines; this is a 10mm Auto magazine. They fit the same frame but feed different cartridges.

Glock G21 Gen4 MOS Slide (OEM, stripped)

  • Fits Glock G21 13-Round Magazine (OEM) · Glock G21 10-Round Magazine (OEM) · ETS G21 13-Round Magazine · ETS G21 18-Round Magazine
  • Won't fit Glock G20 15-Round Magazine (OEM) · Glock G20 10-Round Magazine (OEM) · ETS G20 15-Round Magazine · ETS G20 20-Round Magazine A .45 ACP slide needs G21 magazines; this is a 10mm Auto magazine. They fit the same frame but feed different cartridges.

Slide and Recoil Spring

Gen3 slides take a single spring, Gen4 slides a dual spring.

Glock G20 Gen3 Slide (OEM, stripped)

  • Fits Glock G20/G21 Gen3 Recoil Spring Assembly (OEM) · ISMI G20/G21 Gen3 Stainless Guide Rod & Spring
  • Won't fit Glock G20/G21 Gen4 Recoil Spring Assembly (OEM) This slide takes the Gen3 single-spring recoil assembly; this one is Gen4 dual-spring.

Brownells RMR Cut Slide for Glock 20 Gen3

  • Fits Glock G20/G21 Gen3 Recoil Spring Assembly (OEM) · ISMI G20/G21 Gen3 Stainless Guide Rod & Spring
  • Won't fit Glock G20/G21 Gen4 Recoil Spring Assembly (OEM) This slide takes the Gen3 single-spring recoil assembly; this one is Gen4 dual-spring.

Zaffiri Precision ZPS.2 RMR Cut Slide, Glock 20 Gen3

  • Fits Glock G20/G21 Gen3 Recoil Spring Assembly (OEM) · ISMI G20/G21 Gen3 Stainless Guide Rod & Spring
  • Won't fit Glock G20/G21 Gen4 Recoil Spring Assembly (OEM) This slide takes the Gen3 single-spring recoil assembly; this one is Gen4 dual-spring.

Glock G20 Gen4 Slide (OEM, stripped)

  • Fits Glock G20/G21 Gen4 Recoil Spring Assembly (OEM)
  • Won't fit Glock G20/G21 Gen3 Recoil Spring Assembly (OEM) · ISMI G20/G21 Gen3 Stainless Guide Rod & Spring This slide takes the Gen4 dual-spring recoil assembly; this one is Gen3 single-spring.

Glock G20 Gen4 MOS Slide (OEM, stripped)

  • Fits Glock G20/G21 Gen4 Recoil Spring Assembly (OEM)
  • Won't fit Glock G20/G21 Gen3 Recoil Spring Assembly (OEM) · ISMI G20/G21 Gen3 Stainless Guide Rod & Spring This slide takes the Gen4 dual-spring recoil assembly; this one is Gen3 single-spring.

Glock G21 Gen3 Slide (OEM, stripped)

  • Fits Glock G20/G21 Gen3 Recoil Spring Assembly (OEM) · ISMI G20/G21 Gen3 Stainless Guide Rod & Spring
  • Won't fit Glock G20/G21 Gen4 Recoil Spring Assembly (OEM) This slide takes the Gen3 single-spring recoil assembly; this one is Gen4 dual-spring.

Glock G21 Gen4 Slide (OEM, stripped)

  • Fits Glock G20/G21 Gen4 Recoil Spring Assembly (OEM)
  • Won't fit Glock G20/G21 Gen3 Recoil Spring Assembly (OEM) · ISMI G20/G21 Gen3 Stainless Guide Rod & Spring This slide takes the Gen4 dual-spring recoil assembly; this one is Gen3 single-spring.

Glock G21 Gen4 MOS Slide (OEM, stripped)

  • Fits Glock G20/G21 Gen4 Recoil Spring Assembly (OEM)
  • Won't fit Glock G20/G21 Gen3 Recoil Spring Assembly (OEM) · ISMI G20/G21 Gen3 Stainless Guide Rod & Spring This slide takes the Gen4 dual-spring recoil assembly; this one is Gen3 single-spring.

Barrel and Muzzle Device

Every G20 and G21 barrel we list, checked against each thread protector.

Glock G20 Barrel (OEM)

  • Won't fit Lone Wolf Thread Protector, 9/16x24 This barrel isn't threaded, so the 9/16x24 muzzle device has nothing to screw onto. Choose a threaded barrel.
  • Won't fit Lone Wolf Thread Protector, .578x28 This barrel isn't threaded, so the .578x28 muzzle device has nothing to screw onto. Choose a threaded barrel.

Lone Wolf LWD G20 Barrel, 10mm

  • Won't fit Lone Wolf Thread Protector, 9/16x24 This barrel isn't threaded, so the 9/16x24 muzzle device has nothing to screw onto. Choose a threaded barrel.
  • Won't fit Lone Wolf Thread Protector, .578x28 This barrel isn't threaded, so the .578x28 muzzle device has nothing to screw onto. Choose a threaded barrel.

Lone Wolf AlphaWolf G20 Barrel, 10mm

  • Won't fit Lone Wolf Thread Protector, 9/16x24 This barrel isn't threaded, so the 9/16x24 muzzle device has nothing to screw onto. Choose a threaded barrel.
  • Won't fit Lone Wolf Thread Protector, .578x28 This barrel isn't threaded, so the .578x28 muzzle device has nothing to screw onto. Choose a threaded barrel.

Lone Wolf AlphaWolf G20 Barrel, Threaded 9/16x24

  • Fits Lone Wolf Thread Protector, 9/16x24
  • Won't fit Lone Wolf Thread Protector, .578x28 The barrel is threaded 9/16x24 but the muzzle device is .578x28. They won't screw together.

Lone Wolf AlphaWolf G20 10mm to .40 S&W Conversion Barrel

  • Won't fit Lone Wolf Thread Protector, 9/16x24 This barrel isn't threaded, so the 9/16x24 muzzle device has nothing to screw onto. Choose a threaded barrel.
  • Won't fit Lone Wolf Thread Protector, .578x28 This barrel isn't threaded, so the .578x28 muzzle device has nothing to screw onto. Choose a threaded barrel.

Glock G21 Barrel (OEM)

  • Won't fit Lone Wolf Thread Protector, 9/16x24 This barrel isn't threaded, so the 9/16x24 muzzle device has nothing to screw onto. Choose a threaded barrel.
  • Won't fit Lone Wolf Thread Protector, .578x28 This barrel isn't threaded, so the .578x28 muzzle device has nothing to screw onto. Choose a threaded barrel.

Lone Wolf AlphaWolf G21 Barrel, .45 ACP

  • Won't fit Lone Wolf Thread Protector, 9/16x24 This barrel isn't threaded, so the 9/16x24 muzzle device has nothing to screw onto. Choose a threaded barrel.
  • Won't fit Lone Wolf Thread Protector, .578x28 This barrel isn't threaded, so the .578x28 muzzle device has nothing to screw onto. Choose a threaded barrel.

Lone Wolf LWD G21 Barrel, Threaded .578x28

  • Fits Lone Wolf Thread Protector, .578x28
  • Won't fit Lone Wolf Thread Protector, 9/16x24 The barrel is threaded .578x28 but the muzzle device is 9/16x24. They won't screw together.

Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28

  • Fits Lone Wolf Thread Protector, .578x28
  • Won't fit Lone Wolf Thread Protector, 9/16x24 The barrel is threaded .578x28 but the muzzle device is 9/16x24. They won't screw together.

Parts Worth a Look

Our picks from the Glock 20 / 21 catalog, with the lowest price we list (prices last checked Oct 5, 2026).

Slide

Budget Pick

Glock G20 Gen3 Slide (OEM, stripped)

10mm · Gen3 · No optic cut

Plain factory slide.

$199.99 at GlockStore (sample price)

Best Value

Brownells RMR Cut Slide for Glock 20 Gen3

10mm · Gen3 and 20SF · RMR footprint · 17-4 stainless

Direct-mount RMR cut for about the price of a factory slide.

$219.99 at Brownells (sample price)

Premium Pick

Glock G20 Gen4 MOS Slide (OEM, stripped)

10mm · Gen4 · MOS plate system

Factory optic plates fit most pistol dots.

$279.99 at GlockStore (sample price)

Barrel

Budget Pick

Lone Wolf LWD G20 Barrel, 10mm

10mm · 4.61" · Conventional rifling

Conventional rifling for lead and hard-cast loads, for less than a factory barrel.

$109.95 at Brownells (sample price)

Best Value

Lone Wolf AlphaWolf G20 Barrel, 10mm

10mm · 4.61" · Conventional rifling

Drop-in, and safe for cast hunting loads.

$134.95 at Brownells (sample price)

Premium Pick

Lone Wolf AlphaWolf G21 Barrel, Threaded .578x28

.45 ACP · Threaded .578x28

Suppressor-ready .45 with conventional rifling.

$149.95 at Brownells (sample price)

Start from a Complete Build

Every part already checked to fit. Open one in the builder and swap anything you like.

Budget Glock 20 / 218 parts, $571.30 at the lowest prices we listBest Value Glock 20 / 219 parts, $1,236.41 at the lowest prices we listPremium Glock 20 / 2110 parts, $1,454.36 at the lowest prices we list

Sources

Updated Oct 5, 2026.

More Fit Questions