Glock 43X, 43X MOS, and 48 MOS: How to Pick the Right Red Dot

The 43X MOS and 48 MOS use a Shield RMSc direct-mount cut, not the RMR-footprint cut the G17 MOS uses. The full-size G17 MOS has a four-plate system; the slimline does not. If you already own a Trijicon RMR or another RMR-footprint optic for a G17, it will not drop on a 43X or 48 without an adapter plate. This guide covers what fits the slimline cut directly, what does not, and how to mount it. It also explains the one detail that most "best red dot" lists for these pistols either skip or get wrong — and why picking the right footprint before you buy saves a $200 mistake.

01 ·What is actually on the 43X and 48 MOS slide

The gun is a 9mm with a 3.41-inch barrel, a 10+1 capacity, and a slide cut for a specific compact footprint. That footprint is Shield RMSc. It has four recoil lugs (two at the front, two at the rear) and four 4-40 screws. The slide itself is 0.87 inches wide — narrower than a full-size G17 at 1.00 inches, which is why wider-footprint optics such as an RMR-footprint model will not drop on.

Glock's 2019 press release announcing the optic-ready Slimline models calls this out directly: the slimline cut uses a non-standard MOS footprint and requires specific compact optics such as the Shield RMSc. That is the company's way of saying this is not the same cut as the G17 MOS. If you own a full-size Glock MOS and you are used to the four-plate system, leave that mental model at the door.

What this means in practice: you need an optic that is listed as RMSc. An RMR-footprint optic will not drop on directly — these are all wider-footprint optics — they are all wider than the slimline slide. The CVLIFE optics that do fit are the WildHawk Enclosed and the BearMight, both of which list the RMS / RMSc footprint on the spec page. (If you are shopping CVLIFE for a slimline, those are the two to look at first.)

02 ·43X vs 43X MOS vs 48 MOS — which one and why

All three share a grip, a 10-round magazine, and a 0.87-inch slide width. The differences are the optic cut, the slide length, and the weight.

Spec	43X	43X MOS	48 MOS
Optic ready	No	Yes (direct mount, Shield RMSc)	Yes (direct mount, Shield RMSc)
Barrel	3.41"	3.41"	4.17"
Overall length	6.50"	6.50"	7.28"
Height (with magazine)	5.04"	5.04"	5.04"
Weight (with empty magazine)	18.55 oz	18.55 oz	20.74 oz
Capacity	10+1	10+1	10+1

43X (no MOS) is the budget pick. No optic cut, no plate, no drama. About $50 cheaper than the MOS version. If you have no plans to add a red dot in the next two years, this is the gun.

43X MOS is the same gun with the optic cut. About $50 more. If you are buying new and you might ever want a red dot, get the MOS. The $50 buys you the cut and the cover plate; it does not buy you a red dot, so budget for that separately.

48 MOS is the longer-barrel, slightly heavier sibling. The extra 0.78 inches of slide and 2.2 ounces of weight translate to a longer sight radius and noticeably softer recoil. The 48 is generally easier to shoot well, mostly because of the longer sight radius, and the 0.78 inches of extra slide is invisible under a t-shirt. If you shoot a lot and you want a slimline that feels closer to a compact duty pistol, this is the one.

For a first concealed-carry pistol, the 48 MOS is the safer bet. For appendix carry under light clothing where the shortest possible slide matters, the 43X MOS. Both use the same Shield RMSc cut, so the same optics work for both.

03 ·Five red dots that fit, ranked by use case

If you have a RMR sitting in a drawer and you really want to use it on a 43X MOS, you can — with a third-party plate. The trade-off is a stack height of 0.06 to 0.12 inches, which moves the optic out of the low co-witness position the slimline cut was designed to deliver. For range use, fine. For carry, buy the right footprint.

04 ·How to mount a red dot on a 43X MOS in 5 steps

This is a direct-mount install. No plate, no shim, no fitting. The only thing that can go wrong is the optic not matching the recoil-lug pattern — see step 2.

Remove the cover plate. T8 Torx driver, four screws. Set the cover plate aside; you will not need it once the optic is on.
Confirm the optic is true RMSc or modified RMSc. True RMSc optics engage all four recoil lugs. Modified RMSc optics (Holosun 407K, 507K) only engage the two front lugs. Both fit. They hold zero differently.
Dry-fit and check the lugs. Place the optic on the cut without screws. The two front lugs should drop fully into the front sockets. The two rear lugs (or the rear shelf, on a modified-RMSc optic) should sit flush. If the optic rocks, you have a footprint mismatch — stop and re-verify before torquing any screws.
Apply thread locker and torque. One drop of blue 242 Loctite per screw. The factory screws are 4-40 x 1/4-inch T8. Torque to 12 inch-pounds in a star pattern: front-left, rear-right, front-right, rear-left. True RMSc optics from Shield and Sig land in the 10 to 15 in-lb range, so 12 is a safe middle.
Verify zero at the range. Fire 50 rounds at 7 yards, then 20 rounds at 15. A 43X or 48 MOS with a properly mounted true-RMSc optic should hold zero through 1,000+ rounds of 9mm with no shift. If point of impact drifts more than 2 inches, recheck screw torque and confirm the lugs are fully seated.

05 ·Other slimline carry guns, briefly

Where the 43X and 48 MOS pull ahead is the ecosystem: holsters, magazine extensions, optic cuts, and aftermarket support are all deeper for the Slimline platform than for any of those three. The 48 MOS is the safer bet for a first carry gun, not because the others are bad, but because the parts catalog is bigger.