The Glock Modular Optical System (MOS) ~ Simon Stewart

What is MOS?

Glock 17 Gen 5 MOS FS

By definition the  Glock Modular Optical System (MOS) is a reliable and simple mounting  system engineered by Glock.  MOS facilitates the attachment of a reflex sight (Red Dot) onto the iconic semi-automatic pistol. Ideally, a reflex sight needs a machined platform to mount to the top of a semi-auto slide. Some systems use the machined slot or dovetail where the back sight is mounted as the primary reflex sight mounting point. This does not, however, give the sight the optimum platform size. Consequently the stability it requires to withstand the repeated high g- force cycles it has to cope with is compromised.  Like all compromises, mount stability will affect the accuracy of the sight.

Glock slide milling South Africa.

The MOS system achieves a stable mounting platform to ensure sight accuracy. The slide has a slot machined into it about 48mm long in the Glock factory. This slot then has a cover plate fitted into it – even with matching slide serrations –  that makes the frame look indistinguishable from a standard Gen 4 or 5 at first glance. A question we are are often asked is if Glock slide milling is available in South Africa. Although I have heard of successful attempts at doing this there are many unsuccessful horror stories out there. Exercise extreme caution if you try to go this route and of course bear in mind that all warranties on your Glock will be voided by this action.

Simple Installation

The reflex sight installation is then simple.  Firstly you unscrew and remove the cover plate, position the new plate ( four plates are supplied to fit the most common optics) designed for use with your optic and screw it in position. The MOS plate kit supplied with the pistol. Furthermore it even has Loctite coated screws.  Subsequently attach your optic of choice to the plate with screws, as per your sight manufacturer’s instructions and you’re good to go.

Glock MOS schematic

Four plates are supplied in the kit that will suit the following optics:

  • Plate1 -Eotech/TruGlo/Insight/Meopta
  • Plate2 -Trijicon RMR
  • Plate3- C-More
  • Plate 4 -Leupold Delta Point

If you have an optic, not on the list don’t despair:  many lesser-known sights use a mounting system identical to one of the above.

The Reflex or Red Dot Sight

Augmented Reality (AR) is a term that applies to any display technology capable of overlaying graphical information within the user’s view of the real world. A heads up display is a perfect example and so, of course, is a reflex or “red dot” gunsight.

Origins of the System

Sounds like “Star Wars” technology but in reality, the origins of the reflex sight system date back formally to Dublin, Ireland, 1901. In fact, it possibly even predates this by a century.  A magicians stage trick now known as “The Pepper Effect” used the same principle. Howard Grub – a telescope maker – patented a reflex (reflective) sight in 1901.  A virtual image (read here, red dot) was projected onto an optical sight. This reflex principle of Grub’s was later adopted on fighter aircraft gunsights at the end of WW1 and subsequently on bomb and anti-aircraft sights.

Glock Modular Optical System MOS

Enter the LED

The development of the small energy-efficient light-emitting diodes (LEDs) in the seventies and light gathering fiber optics allowed for the future miniaturisation of reflex sights to a degree where they are now available for handguns. It is worth mentioning that the next evolutionary step for the reflex sight design is the use of a holographic projection rather than a projected LED or another light source such as a tritium beta lamp. The miniaturisation of lasers has made these reflex style holographic sights possible. The technology is highly patented though and at present only available from Eotech.

Advantages of the reflex sight.

The reflex sight has many advantages over a conventional set of iron sights. For me the most important is that a reflex optic places the dot in the same focal plane as the target, so both are in focus. Unlike when using conventional sights where you keep the front sight sharp and the target blurry – the red dot sight lets you keep focused on the action: extremely useful when the going gets tough. A further benefit is that there is virtually no parallax error on a well-designed sight due to the use of collimated (light that goes in straight parallel lines) light. To the shooter, MOS means that the eye does not have to be perfectly aligned with the sight at all times to achieve optimum accuracy. Useful in a gunfight? – I’ll say.

Truglo RMR Red Dot Gunsight
TruGlo RMR Red Dot Sight

Reflex sights have the following advantages:

⁃ close to medium range targeting easier.
⁃ both eyes remain open instead of one eye closed like conventional sights
⁃ the reticle of the sight can be viewed at any reasonable distance
⁃ no set eye relief aiding in faster recognition of the reticle and consequently target

And it all began with smoke and mirrors…..

MOA and Red Dot Sights

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