Overall, the MOS is fine. The vast majority of people won’t have an issue with it. Yes, there can be problems, but they’re few and far-between and the vast majority of people who bring these issues up wind up referencing the same few cases posted online. It’s not some catastrophic failure of a system, and there are tens of thousands of these guns out on the market and being used by police, and the actual documented failures rest in an extreme minority. If this is a concern, there is a solution.
The chief issue lies with the adapter plates being made by MIM, so when a plate is cast, it cannot be machined to specific tolerances. As such, there are plates that are slightly warped. When the plate is warped, it causes an issue with screw thread engagement and actually leverages opposing screws upward, which then can lead to screws breaking or loosening. Glock could solve this by switching using plates made out of machined ordnance grade steel.
The second issue people make light of are cheap screws. The screws could be better, which is why it’s easy enough to go online and get some replacement screws from McMaster-Carr. The mounting screws you want are part# 94414A712, which are M3x0.50mm x 6mm Torx Plus screws, and have a tensile strength of 140,000psi, with a Rockwell38 hardness.
The thinness of the plate causes the third issue, which is minimal thread engagement for the screws mounting the optic to the plate.
So to solve all of this, it’s as easy as getting a replacement plate from an aftermarket company. The best plates are from CHPWS, as they solve every problem. They are machined aluminum, fully flush and square, have upgraded screws, and the optic mounting hole doubles as additional recoil bosses, and they double the amount of thread engagement with the optic mount screws. For most optics, the CHPWS is superior. TangoDown has an outstanding ACRO plate made out of ordnance grade steel, which I’m currently running on a G45 MOS. The Glock MOS issue is simply the plates and screws.
As part of an agency T&E process, I ran factory mounting components on a Gen4 G17 MOS with a RMR RM06 for 2.5 years, and I had zero issues. We ran 4 Glocks in different configurations with factory MOS components, and all were problem-free for the test period, which ranged from 1.5-2.5 years. I beat the piss out of the optic, and I did pretty much everything to it that Aaron Cowan does to his, minus the drop testing. The key here was that I was fortunate to have a factory plate that sat flush and wasn’t warped.
Direct-mounting is the most reliable, and the benefit is that you have perfect tolerances and a setup that is the most durable and accurate. The downside is obviously that the slide is then slaved to a single optic footprint, and changing to a different optic footprint will mean getting either a new gun, or at least a new slide. If you run an RMR, direct mounting means you won’t have to rezero the optic after a battery change.
The increased mounting height can be an issue for the Delta Point Pro due to it having a very high main body housing, but with all other optics, the slight increase in height is really a non-issue.
If you have an MOS and you have concerns about the durability of the whole system, you can always have it machined by Agency Arms for their AOS plate system. I have another Gen4 G17 that has the AOS plate system on it, and the AOS is the superior plate system.