How To Install A Red Dot Using the Glock MOS System

Glock pistols are some of the most ubiquitous and popular on the planet. Ranging from Large duty-sized pistols like the Glock 17 down to the incredibly small and concealable Glock 43 and 42, there is a Glock pistol, and Glock Holster to fit virtually any situation out there. It’s hard to believe that it’s already been almost 10 years since the announcement of the Glock MOS system back in 2015. Red Dot optics on pistols have exploded in popularity since then and have only grown more popular, branching out of their origins on rifles and making their way onto not just competition pistols, but revolvers and micro-compact carry pistols as well. All Glock OEM pistols make use of the MOS or “Modular Optic System.” This system makes use of a series of proprietary numbered plates to adapt the Glock MOS’ slide from simple iron sights to any of the popular red dot patterns out there including RMR, RMSc, Delta Point Pro, and more. The MOS system is fairly straightforward to use but can be easy to mess up for those who have never tried to mount an optic to a pistol before. So today, we’ll be going over how to mount a red dot to your pistol using the Glock MOS system.

Glock equipped with the red dot sight
Glock equipped with the red dot sight


Materials and Tools You Should Have

Glock typically includes only a hex wrench, some thread locker, and some initial hardware with their MOS plate system. If your pistol came with just one MOS plate, you’ll likely also have the same materials. Complete Glock MOS plate systems can be purchased and they are about $35 or so online from various retailers. This kit will include mounting hardware, and four different plates for C-MORE, Leupold/Eotech, Trijicon/Holosun/Ameriglo, and Docter/Meopta/Insight/Vortex/Burris red dot optics. The only major difference you need to watch out for when purchasing your initial MOS kit is to make sure you purchase the kit for the right width of your handgun’s slide. There are two different kits, one for pistols in MOS configuration

Glock MOS plate system
Glock MOS plate system

with a slide width of 25.5 mm / 1.00 in, and one for Glock pistols in MOS configuration with a slide width 27.5 mm / 1.06 in and slide width 28.5 mm / 1.12 in. Other than the MOS mounting kit, its included tools/hardware, and your optic, it might also be handy to have the following tools around to make the installation process a bit easier.

Torque wrench along with thread locker
Torque wrench along with thread locker
  • Torque Wrench/Limiter
  • Isopropyl Alcohol or Brake Cleaner
  • Pistol Vice Block or Slide Block (Something to secure the slide in while you work on it)


Preparing Your Pistol

Before working on your handgun at all you should always make sure your work surface is clean and clear from clutter, and that you don’t have any live ammunition sitting around your workspace. After clearing and unloading your pistol, you can field strip it by removing the slide, barrel and recoil spring to make it easier to work on a flat surface or inside of a vice block.

Glock optic cover plate
Glock optic cover plate

MOS-equipped Glock pistols usually ship with an optic cover plate that can easily be removed using only the torx or hex wrench included with your pistol. You’ll want to start by removing this optic cover plate, and its hardware and set them back with your pistol case or wherever you keep your spare parts for your Glock.

MOS plates for different red dot sights
MOS plates for different red dot sights

Once the MOS cover plate has been removed, you should then make sure that the surface of the Glock’s slide is clean from any debris that may have found its way in there. You can follow up by wiping it clean with brake cleaner or isopropyl alcohol to remove any oils or fouling - this will ensure that your MOS plate has the best mating surface to the pistol’s slide when we go to install it. After this step, we’ll want to take some time to prepare your MOS plate.


Preparing the MOS Plate

By now you should have removed the correct MOS plate for your optic and inspected it to make sure that it is not bent - as thick as the steel on MOS plates is, it is possible for them to get bent during shipping if the shipper did not take care to properly protect it. If it lies flat on the back of the pistol’s slide without any issue, you should be good to go. Take this time to give the MOS plate the same treatment as the slide and wipe it clean from any oils and debris in preparation for mounting to the pistol’s slide.

Glock MOS plate
Glock MOS plate

Next, you’ll want to make sure you have the correct mounting hardware. MOS plate mounting hardware is shorter than your optic’s hardware and should have come with both your pistol and or the MOS plate kit you purchased. The hardware should be carefully inspected to make sure that it has a fresh thread locker patch on it. If you get the feeling that your mounting kit hardware has been sitting around for a long time (usually it has) you’ll want to have some additional threadlocker on hand for when you go to mount the hardware. Before applying any new thread locker, test-fit the hardware into the Glock pistol’s slide to make sure that the hardware fastens freely without any cross-threading. Once you’re satisfied your MOS plate and hardware are good to go, it’s time to mount the plate to the slide.

Screws / Hardware for Glock MOS optics
Screws / Hardware for Glock MOS optics


Mounting the Plate to the Slide

Once both pieces are prepped you should first place the MOS plate on the slide - there is only one way to orient the plate so if it doesn’t fit one way, simply flip it 180 degrees and it should go right in! Next, you’ll want to apply a small amount of medium-strength thread locker onto the threads of the hardware - usually, a single small drop should suffice.

How to apply thread locker
How to apply thread locker

Next, insert the hardware and then slowly hand-tighten it using only your fingers, alternating between the two pieces of hardware to make sure the plate is tightened evenly. Once the plate is snug by hand tightening Glock officially says you’re done securing the optic to the pistol. However, in my experience, I’ve found it’s better to secure the plate to the slide using about 13.3 in/lb. in or 1.5 Nm and a torque wrench. I like this method of securing the optic to the plate better as it has two benefits.

Using torque wrench on the Glock slide
Using torque wrench on the Glock slide

First, you will have very repeatable and stronger fastening to the pistol slide, secondly, when you go to remove and re-seat the plate for whatever reason, any optic that goes back on the plate and slide is more likely to have less of a drift of zero - this is incredibly handy for optics that have to be removed to replace their battery. Once the plate has been properly torqued down to the slide, you’re ready to mount the optic to the plate.

Properly fitted MOS plate for optics
Properly fitted MOS plate for optics


Mounting the Optic to the Plate

Repeat the cleaning process that you did for the slide and MOS plate for the bottom of the optic, if you’re using an optic like the RMR that requires a weather plate on the bottom of it, make sure that it's the thing being cleaned and not the interior of the RMR! Once again inspect and test fit your optic mounting hardware and prepare it with a small amount of medium thread locker.

Mounting the RDS to the plate
Mounting the RDS to the plate

Mount the optic to the MOS plate and slide and then insert and tighten the optic’s hardware as before taking special care to observe how loose the optic is when the tightening gets difficult, if you still see the head of your hardware not sitting flush with the optic’s body and its already difficult to tighten the hardware, STOP! This likely means that you’ve selected the wrong hardware for your optic’s body height and need to find the correct hardware.

Properly mounted RDS on a Glock slide
Properly mounted RDS on a Glock slide

If everything is going smoothly, hand-tighten the hardware, and then tighten the hardware using a torque limiter or wrench to about 15-18 in/lb or 1.5 Nm - this is a torque setting that I have found works best with quality hardware - cheap or knockoff mounting hardware often fails before this torque spec so it’s in your best interest to purchase high-quality aftermarket, or genuine OEM MOS and optic hardware.

Defender XL optic mounted on a Glock 45 MOS slide
Defender XL optic mounted on a Glock 45 MOS slide


Final Steps

Once your optic and plate are securely mounted to your pistol slide, a final step I like to take is to use either a scribe or a very fine high-visibility white or silver market to put witness marks on your optic hardware. What this does is give you a quick visual indicator of the position of your mounting hardware. This could save you from losing both the optic and its hardware before they fly off your gun so you can go back and reapply the thread locker and proper torque.

Safety markings on a hardware
Safety markings on a hardware

You’ll next want to reassemble your pistol and then test fit it empty inside of your Glock pistol holster. Many modern Kydex holsters and leather Glock holsters are capable of working with even the largest modern red dots like the Holosun 507 COMP, Trijicon RMR HD2, and the Vortex Defender-XL. If yours isn’t fitting, Falco Holsters has a wide selection of optics-ready & even light-bearing Glock holsters to choose from that will fit your Glock MOS pistol and optic combination just fine.

Luke Cuenco

Luke Cuenco

Writer

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