Glock MOS or MOS-K? Check before you buy the optic
Glock now makes two slim slide cuts and they take different optics. How to tell which one you have, and whether a 507K really fits a 43X.
If you own a Glock 43X MOS or a 48 MOS and you are shopping for a red dot, there is one thing to check before you spend a penny. Glock now makes two different slim slide cuts, and most of the advice online was written before the second one existed.
How to tell which one you have
Glock’s own instruction is refreshingly concrete: look at the item number sticker on the handle of your pistol case. The original cut shows MOS near the end of the item number. The newer cut shows MOSK.
That is the whole test, and it matters more than anything else on this page. Two 43X MOS pistols sitting next to each other on a shelf can take different optics.
What each cut takes
- Standard Slimline MOS. Cut for the Shield RMSc footprint, and it uses M4 screws. No adapter plate — the optic mounts straight to the slide.
- MOS-K. Modified to accept either the Holosun K footprint or RMSc, again with no plate. Glock says to use the M3 screws supplied by the optic manufacturer.
Note the screw difference between the two cuts. That alone is a good reason not to reuse hardware from a previous install.
So will a Holosun 507K fit a 43X MOS?
This is the question everyone actually types, and the honest answer is: it depends which cut that specific pistol has, and we should look at it before you buy.
If it is a MOS-K gun, yes — directly, no plate. Glock says so.
If it is a standard Slimline MOS cut, the weight of the evidence says you need an adapter. Glock cut that slide for RMSc. Holosun calls the 507K a “K” footprint on its own product page and treats K as a separate pattern — so much so that Holosun includes an RMSc-to-K adapter plate in the box with its EPS Carry. And C&H Precision manufactures a dedicated 43X/48 MOS-to-407K/507K plate, which nobody tools up for a combination that already fits.
The clincher, to us, is simply that Glock created the MOS-K cut in the first place. If K optics already dropped onto the standard cut, there would have been no reason to machine a second pattern.
You will find a video or two showing someone running a 507K on a standard cut without a plate. We are not going to call anyone a liar — the 507K ships with both M3 and M4 screws, so it can be threaded down. Whether it is properly supported on every lug is a different question, and it is your slide.
The full-size guns, which also changed
Two things worth knowing if you are shopping a 17, 19, 45 or 47:
- Gen4 and Gen5 MOS now ship with a single adapter plate, not the old four-plate set. Glock says that plate covers Trijicon, Ameriglo and Holosun — except the 509.
- Gen6 ships three plates, and Glock publishes the mapping: 02 for Trijicon RMR, RCR, RMR HD and SRO plus Holosun 508T, 407C, 507C and 507Comp; 03 for C-More; 04 for Vortex Defender ST and XL, EOTech EFLX and Leupold DeltaPoint Pro.
If someone tells you your MOS gun comes with four plates, they are describing a box Glock stopped shipping.
One rule Glock states outright
Do not use the optic cover plate screws to mount your optic. Those screws exist to hold the blanking plate on, and Glock says explicitly that they are not for the optic. They are a different length, and this is a common and expensive substitution.
The direct-cut future
Worth knowing that Glock now also offers slides cut for Aimpoint’s A-CUT, which mounts the optic directly using a wedge-locking dovetail rather than screws taking the lateral load. No plate, no adapter. It is the clearest sign yet that the industry is trying to get out of the plate business.
Bring the case sticker
Genuinely — bring the pistol, and if you still have the case, bring that too. We will read the item number, look at the cut, and put the optic on the slide before you buy it. We stock Holosun, Vortex, Sightmark, Leupold and Trijicon, and we have Glocks here to check against.
Route 112 in Medford. It takes five minutes and it saves the wrong purchase.