Will a Holosun 507K fit a Glock 43X MOS?

This is the fitment question that catches people out, because the screws line up. The Glock 43X MOS and 48 MOS slides are cut in the Shield RMSc pattern, and the Holosun 507K uses the same screw spacing — so the optic will bolt on and look mounted. Whether it is actually seated is a different question, and it depends on the posts.

The factory Slim MOS cut has four locating posts machined into the slide, two at the front and two at the rear. They carry recoil so the screws are not the only thing holding the optic in place. A true RMSc-footprint optic has four matching recesses in its underside and drops over all four. Holosun K-series optics have two, and shallower — so on a four-post cut, two posts have nowhere to go and hold the optic proud of the slide. It rocks, and the screws end up clamping it onto two hard points instead of a flat surface.

Check your own slide before ordering anything. Take the cover plate off and count the posts. Four posts means you need an adapter plate cut for a Holosun K optic, or a slide machined to remove the extra pair. Two posts means the optic can seat directly. Optics that genuinely are RMSc — the Shield RMSc and SMSc among them — clear all four and mount without a plate either way.