Which red dots fit a Glock 20 or 21?
You can put a red dot on a Glock 20 or 21 two ways: a factory MOS slide with an adapter plate, or an aftermarket slide milled for one footprint. Either way the optic has to match the plate or the cut.
The Short Answer
- A slide with no optic cut can't take a red dot unless it is milled.
- An RMR-cut G20 slide takes RMR-footprint optics directly, such as the Holosun 507C and 407C and the Trijicon RMR and SRO. Other footprints need an adapter plate.
- A G20 or G21 MOS slide always uses a plate, and it takes Glock's large-frame MOS plates for 10mm and .45, not the 9mm set.
- The Aimpoint Acro uses Aimpoint's own MOS plate.
- With a dot on the slide, suppressor-height sights let you aim through the window if the dot fails. Standard-height sights sit below it.
Slide and Optic
Every G20 and G21 slide we list, checked against every optic.
Glock G20 Gen3 Slide (OEM, stripped)
- Won't fit Holosun HS507C X2 · Trijicon RMR Type 2, 3.25 MOA · Trijicon SRO 2.5 MOA · Aimpoint Acro P-2 · Holosun HS407C X2 This slide has no optic cut. Choose a MOS or optic-cut slide, or skip the optic.
Brownells RMR Cut Slide for Glock 20 Gen3
- Fits Holosun HS507C X2 · Trijicon RMR Type 2, 3.25 MOA · Trijicon SRO 2.5 MOA · Holosun HS407C X2
- Check Aimpoint Acro P-2 The slide is cut for the RMR footprint and this optic uses ACRO. You need an adapter plate.
Zaffiri Precision ZPS.2 RMR Cut Slide, Glock 20 Gen3
- Fits Holosun HS507C X2 · Trijicon RMR Type 2, 3.25 MOA · Trijicon SRO 2.5 MOA · Holosun HS407C X2
- Check Aimpoint Acro P-2 The slide is cut for the RMR footprint and this optic uses ACRO. You need an adapter plate.
Glock G20 Gen4 Slide (OEM, stripped)
- Won't fit Holosun HS507C X2 · Trijicon RMR Type 2, 3.25 MOA · Trijicon SRO 2.5 MOA · Aimpoint Acro P-2 · Holosun HS407C X2 This slide has no optic cut. Choose a MOS or optic-cut slide, or skip the optic.
Glock G20 Gen4 MOS Slide (OEM, stripped)
- Check Holosun HS507C X2 · Trijicon RMR Type 2, 3.25 MOA · Trijicon SRO 2.5 MOA · Holosun HS407C X2 10mm and .45 MOS slides take Glock's large-frame MOS plates, not the 9mm set. Get the large-frame plate for the RMR footprint.
- Check Aimpoint Acro P-2 10mm and .45 MOS slides take Glock's large-frame MOS plates, not the 9mm set. Get the large-frame plate for the ACRO footprint (Aimpoint sells its own MOS plate for the Acro).
Glock G21 Gen3 Slide (OEM, stripped)
- Won't fit Holosun HS507C X2 · Trijicon RMR Type 2, 3.25 MOA · Trijicon SRO 2.5 MOA · Aimpoint Acro P-2 · Holosun HS407C X2 This slide has no optic cut. Choose a MOS or optic-cut slide, or skip the optic.
Glock G21 Gen4 Slide (OEM, stripped)
- Won't fit Holosun HS507C X2 · Trijicon RMR Type 2, 3.25 MOA · Trijicon SRO 2.5 MOA · Aimpoint Acro P-2 · Holosun HS407C X2 This slide has no optic cut. Choose a MOS or optic-cut slide, or skip the optic.
Glock G21 Gen4 MOS Slide (OEM, stripped)
- Check Holosun HS507C X2 · Trijicon RMR Type 2, 3.25 MOA · Trijicon SRO 2.5 MOA · Holosun HS407C X2 10mm and .45 MOS slides take Glock's large-frame MOS plates, not the 9mm set. Get the large-frame plate for the RMR footprint.
- Check Aimpoint Acro P-2 10mm and .45 MOS slides take Glock's large-frame MOS plates, not the 9mm set. Get the large-frame plate for the ACRO footprint (Aimpoint sells its own MOS plate for the Acro).
Optic and Iron Sights
Whether the irons show through the window.
Holosun HS507C X2
- Fits Trijicon Bright & Tough Suppressor Night Sights, Glock Large Frame
- Fits, with a note Glock OEM Polymer Sights · Trijicon HD XR Night Sights, Glock Large Frame Standard-height sights sit below the dot and won't co-witness. Suppressor-height sights let you aim through the window if it fails.
Trijicon RMR Type 2, 3.25 MOA
- Fits Trijicon Bright & Tough Suppressor Night Sights, Glock Large Frame
- Fits, with a note Glock OEM Polymer Sights · Trijicon HD XR Night Sights, Glock Large Frame Standard-height sights sit below the dot and won't co-witness. Suppressor-height sights let you aim through the window if it fails.
Trijicon SRO 2.5 MOA
- Fits Trijicon Bright & Tough Suppressor Night Sights, Glock Large Frame
- Fits, with a note Glock OEM Polymer Sights · Trijicon HD XR Night Sights, Glock Large Frame Standard-height sights sit below the dot and won't co-witness. Suppressor-height sights let you aim through the window if it fails.
Aimpoint Acro P-2
- Fits Trijicon Bright & Tough Suppressor Night Sights, Glock Large Frame
- Fits, with a note Glock OEM Polymer Sights · Trijicon HD XR Night Sights, Glock Large Frame Standard-height sights sit below the dot and won't co-witness. Suppressor-height sights let you aim through the window if it fails.
Holosun HS407C X2
- Fits Trijicon Bright & Tough Suppressor Night Sights, Glock Large Frame
- Fits, with a note Glock OEM Polymer Sights · Trijicon HD XR Night Sights, Glock Large Frame Standard-height sights sit below the dot and won't co-witness. Suppressor-height sights let you aim through the window if it fails.
Parts Worth a Look
Our picks from the Glock 20 / 21 catalog, with the lowest price we list (prices last checked Oct 5, 2026).
Optic
Best Value
Holosun HS507C X2
RMR footprint · Multi-reticle · Solar
Feature-rich and reliable at half the price of an RMR.
$299.99 at Primary Arms (sample price)
Premium Pick
Trijicon RMR Type 2, 3.25 MOA
RMR footprint · Duty-grade
Holds up to full-power 10mm recoil.
$439.99 at OpticsPlanet (sample price)
Sights
Budget Pick
Glock OEM Polymer Sights
Standard height · U-notch rear
Factory sights. Fine to start, easy to replace.
$9.99 at GlockStore (sample price)
Best Value
Trijicon Bright & Tough Suppressor Night Sights, Glock Large Frame
Suppressor height · Tritium
Tall enough to co-witness with a dot and clear a suppressor.
$121.52 at OpticsPlanet (sample price)
Start from a Complete Build
Every part already checked to fit. Open one in the builder and swap anything you like.
Sources
Updated Oct 5, 2026.