If you own a Glock MOS full-size or compact model (excluding the 43X and 48 MOS) and are curious about which Holosun optics will co-witness with your iron sights, we’ve tested each one to demonstrate how they align.
Adapter Plates Used
The Glock MOS mounting platform comes with a proprietary cut slide adapter plates. For most of these you will use adapter plate #2 - which converts the MOS to a RMR footprint. If you do not have the #2 adapter plate from Glock, or you want to install a K footprint optics or direct mount a 509T, check out our adapter plate finder for the Glock here.
Models That Will Co-Witness
The red dots in this article will co-witness these Glock MOS models with standard iron sights:
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Will Fit ✔️
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Will Fit ✔️
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Will Not Fit ❌
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RMR Footprint Optics
Let’s kick things off with the RMR footprint optics, including the 407C, 407A3, 507C, 507C ACSS, 507 Comp, 508T, and the 509T. As mentioned above, Glock makes it easy and gives you a #2 plate in the box. However, with all of these RMR optics, you won't be able to see any of the iron sights.
407C / 407A3 / 507C / 507C ACSS / 507 COMP / 508T
The 407C, 507C, 508T, and all RMR optic variants do not co-witness with the factory Glock #2 plate that comes with the pistol. We also tested three other third-party plates from reputable manufacturers, including CHPWS, Calculated Kinetics, and DPP. Unfortunately, none of these plates were thin enough to offset the higher deck heights associated with RMR optics. As a result, standard iron sights will not align properly with these optics. If you want to achieve co-witness with an RMR optic, you'll need to upgrade to taller iron sights specifically designed to work with the increased height of these optics.
509T
The 509T, like the other optics we've discussed, shares the RMR footprint, meaning you could technically use the included RMR plate and stack the 509T plate on top to mount it. However, that’s not the best solution. Instead, we recommend using the Calculated Kinetics 509T plate, which is specifically designed for the Glock MOS. This method avoids the need for two stacked plates, resulting in a lower deck height. Unfortunately, even with this single plate, the 509T’s deck height is still too high to co-witness with the standard iron sights.
SCS MOS - Best Co-Witness
Holosun designed the SCS MOS specifically to mount directly onto the Glock MOS system without requiring any additional plates. This is the only optic in Holosun's lineup that fits seamlessly onto the MOS platform, providing an immediate co-witness advantage. Its exceptionally low deck height further enhances co-witnessing with the factory iron sights. If you're looking for an optic that co-witnesses without needing to change your iron sights, this is the one to choose. Check out our detailed write-up of the SCS MOS to learn more about this optic.
K Footprint Optics
Now let’s move on to the K footprint optics from Holosun, which include the 407K, 507K, 507K ACSS, SCS Carry, EPS Carry, and the full-sized EPS. All of these optics require an adapter plate for installation. Again we tested plates from three manufacturers—CHPWS, Calculated Kinetics, and DPP—to determine which one offers the best co-witness. The DPP plate turned out to be the thinnest, so we selected it for this setup.
However, because an adapter plate is still necessary, achieving an ideal co-witness is challenging. And instead of relying on the irons sights on the slide, we rely on the rear iron sights built into the K series optics for co-witnessing.
It's important to note that the full-sized EPS lacks built-in rear iron sights, so it will not co-witness.
Additionally, the SCS Carry is incompatible with the DPP plate, and the other plates are too thick, so it also won't co-witness.
This leaves us with four potential co-witness options: the 407K, 507K, 507K ACSS, and EPS Carry.
Among these, I found that the EPS Carry provided the best co-witness, although it only offers a glimpse of the front iron sights. Nonetheless, it is still functional. Above is a picture illustrating this setup.
Conclusion
And there you have it—these are all the co-witnessing options for the Glock MOS with Holosun optics. The SCS MOS stands out as the best choice if you want to achieve co-witnessing without the need for new iron sights. Some of the K footprint optics will co-witness, while none of the RMR optics will. Keep in mind that co-witnessing is just one factor in selecting the right optic, so take your time to find the one that best suits your needs.
