Modifying the Glock 43x & 48 MOS for Holosun K Optics [2025]

I recently picked up a Glock 43x MOS slide from Sean here at Primer Peak. He’s testing the Aimpoint COA Glock, and had a spare slide to offload. He offered it to me, and I chose to buy. However, I’m not a huge fan of the slimline MOS cut. It’s really just an RMSc optic cut, which means that it does not directly work with the Holosun K footprint. On the Glock MOS, you’ve gotta use an adapter plate to use the Holosun K optics (407k/507k), and I don’t like that. The RMSc footprint and the Holosun are very similar, and you can modify the slide to direct mount a Holosun. Well, I did it, and I want to go over the details on how it did it.

Note: While this reads like a guide, I’d recommend having a professional do this work for you. I have a workshop and tools for doing the work, and have competently modified many guns over the years. If you are not confident or skilled enough for this job, I would advise against it, as we are permanently modifying the slide. If you do feel confident in doing this work, make sure to measure twice before cutting! Oh, and you’ll void your Glock warranty with this, so be aware of that.  

Why Modify the Slimline MOS?

There’s a big reason to modify the Slimline MOS to directly fit with the Holosun K footprint: Height. An adapter plate on MOS to work with the Holosun optics adds quite a bit of height, close to 3-4mm depending on your plate. This might not seem like a lot, but it is noticeable when doing dryfire or out shooting. The lower profile will also slightly shrink a spot on the gun for you to need to conceal.

The factory height is also too tall for using standard 43x/48 sights as BUIS. If you modify the optic cut, standard sights will co-witness through a Holosun. This could save you some money, as the OEM sights would work as BUIS. That said, the plastic Glock sights aren’t known for their durability, so I’d still recommend metal ones.

What are the differences between the Glock silmline MOS cut (RMSc) and the Holosun K?

RMSc (Slimline MOS) to Holosun K

They’re both pretty similar, but it comes down to the recoil bosses.

Recoil bosses are small posts that go inside of an optic, and help to keep it from moving under recoil. On the RMSc footprint, we have 4 equal-height recoil bosses. Two at the front of the cut, and two at the back.

On the Holosun K, we only have two, and they’re at the front. They’re about half the height of the ones on the RMSc footprint.

Aside from the recoil bosses, every other part of the cut is the same. If we remove the rear bosses, and chop the front ones down by half, we can just directly mount a Holosun K optic onto the MOS slide. The screw holes are the same, we’d just need screws that are the right size to work (I’ll link the ones I use down below).

If we modify the slide to directly work with the K footprint, it will still work directly with the RMSc optics too. Backwards compatibility could be great!

If you decide to do this work, there is some preparation to do first.

Chopping the MOS – Preparation

Before going whole-hog into cutting on your gun, we should do some preparatory work. Understanding the job, and getting your workspace ready are key for success here.

Understand the Job

To cut the MOS down to work with Holosun K optics, we need to understand the job at hand. Simply put, we’re cutting down the two rear bosses completely, and trimming the front ones down by about half. We’re not trying to go cut into the slide itself, just to remove those rear bosses, and trim the front ones.

The time commitment to the job isn’t very long, but it’s obviously dictated by the pace that you can go. For my first MOS cut, it took me about an hour and a half, but I was taking breaks to check optic fitment. For slide #2, it was about 45 minutes of work, as I had a much better idea of how much material I needed to remove.

If you get your workspace ready for the job, it’ll go faster. I like to start with a (relatively) clean workspace, and organizational bins to put components from the gun into. For this job, I would recommend partially stripping your slide. You don’t need to do it, but you’ll get some metal shavings into the optic screw hole that goes into the extractor channel, and having the extractor plunger out will make it easier to clean it up.

What tools will you need for this?

Tools You’ll Need

There are a handful of tools that will make this job easier.

• Vice
• Good Lighting
• Painters Tape or Electrical Tape
• Permanent Marker
• Bastard File
• Needle File
• Sandpaper or Sanding Stones
• Mineral Spirits or Alcohol
• Cold Blue
• Q-Tips
• Water & Paper Towel

The vice and lighting are pretty basic workbench tools. The vice will hold the slide while you work on it, and the lighting will let you see what you’re doing. The tape comes into play for protecting the slide from accidental nicks, and the marker is for marking the posts to see material removed.

For the actual cutting, I’d recommend a bastard file and some needle files. I know that some folks would probably opt for something like a Dremel here, but I find that hand tools will give you more control, and have a much lower chance of screwing something up. For all of their faults, Harbor Freight’s files work pretty good, and I used them for my work.

For finishing work, I’d recommend a set of thin sanding stones or sandpaper. This is not necessary per se, but it helps to make the cut areas a lot smoother. For refinishing, I recommend a cleaner (mineral spirits or alcohol), and the Birchwood Casey Cold Blue, (or their Super Blue). I find that the Super Blue tends to be a bit more durable, and have a more attractive color too. The Q-Tips are for applying it, and the water and towel is to deactivate the solution from rusting your slide.

It may seem like a lot of tools, but none of them are specific to this job, and they will come in handy if you do any other basic work on your guns.

Ok, so we’ve understood the job ahead, and have our tools ready. How do we actually do the job?

Doing the Job

The actual job is pretty simple. I’ll outline the steps, and give you some detail where it is pertinent.

1. Take the slide off of the frame, and strip the guts out of the slide (striker assembly, extractor plunger). The actual extractor and fining pin block are fine to stay in the slide, as we aren’t working near them.
2.  Tape off areas on the rear of the slide that you do not want to touch with your file. I put a piece in between the front and rear bosses, as that is the most likely spot to catch with your file.
3. Chuck your slide into your vice (padded vice jaws are very handy here).
4. Begin filing on the rear recoil bosses, starting with your bastard file. Once they’ve gotten fairly low, I would switch to the needle file to bring them down.
5. Once the rear bosses are nearly fully cut down, switch to the sanding stone or sandpaper for finish prep.
6. From here, start cutting on the front bosses with your files. Remember that we’re cutting them down to about half height, so be extra careful to take your time. This is the time to test fit the optic to the gun, to see if the bosses sit flush into the recesses in the optic.
7. Once the bosses are cut down to size, smooth them out with your sanding stone or paper.
8. Clean the bosses/area cut with your chemical cleaner. Be sure to pick any debris out of the screw hole that threads into the extractor channel.
9. Once the area is dry, begin to apply your cold blue. Follow the directions on the bottle, which generally states to apply and let sit for 30 seconds, then to wipe down with a wetted towel.
10. Once the bluing is sufficiently down, lightly oil the cut. Your work is finished!

How about some pointers for this work?

Pointers

I won’t sugarcoat it, this is a pretty simple job. That said, take your time. Rushing any sort of gunsmithing work will just lead to a subpar end result. We want to avoid that. Much like my discussion on stippling frames earlier in the year, there is no need to rush. Don’t stonewall yourself into needing to do this job in XYZ amount of time. This is a project that you can complete in a few hours, and at basically every step of the process (aside from bluing), you can drop it and pick it up at a later time.

This is a permanent modification to your slide, so don’t be afraid to go slow. If you cut your rear bosses down by half and decide to switch to a more precise needle file, go for it! Going hand-in-hand with the “no rushing”, taking your time and using the tool that feels best will lead to the best results. Test fit your optic too, as you’ll be able to see when you’ve cut enough of the bosses away with that.

The Verdict

I chopped the bosses on my 43x slide pretty quickly after getting it. I have no regrets on this. It has significantly improved my enjoyment on the gun. I ended up doing the same slide work to a friend’s G48 recently too, and she likes it much more. What else is there to say? Don’t fear chopping your optic cut down!

As a note, this was written right as Glock started to roll out new SKUs of slim guns with ZEV-post style optic cuts. Those should work on RMSc and Holosun K cuts directly, but there are still leagues more of the older MOS cut guns out on the market. As an aside, I don’t really like ZEV posts (requires thinner screws and can have compatibility issues with certain optics), and would rather chop an old MOS gun down than to be stuck with the ZEV posts. Recoil bosses that are also your screw holes can work, but on an optic cut as small as this, I just don’t fancy it.

Video Work

While writing is my primary way to provide you with information, I’m also making videos over on YouTube. You can find that work over at my channel, linked here.

Additional Reading, Patreon Link, & Thanks

Want to start modifying and working on your guns? Check out a few of our articles that cover those topics!

• How to Paint Iron Sights [2025] 
• OT Defense Stippling Kit Review [2025]
• Battle Werx Slide Milling Review [2025]

If you’d like to support me on Patreon, I’ve got the link for that here. Nearly everything that I do on Primer Peak is paid for out of my own pocket, and my content is not shilled or driven by manufacturers or companies. If you decide to donate, I’d really appreciate it, as it would allow for me to continue to bring you quality work.

I’d like to thank my YouTube channel members, channel donors, and Patrons too. Your support allows me to make more works, and to do it without the hand of a company pushing me!