Installing Your Pistol Mounted Optic

When law enforcement and the military units began placing red dot sights on their rifles and submachine guns, the benefits of faster target acquisition and accuracy were quickly realized. As a natural progression, shooters began to explore the use of red dot sights on pistols. However, the large size of the original optics and awkward mounting options slowed their adoption. As advancements in electronics minuturized the sights, engineers became motivated to develop the means to securely mount these sights on pistol slides. Once these hurdles were overcome, the industry exploded to meet the demand of the government, competition, and concealed carriers.

Optic manufacturers began to also offer these sights in various configurations. Consumers now have the option of a reticle (dot with a large circle) instead of a solitary dot. In addition, the projected dot is no longer confined to red; manufacturers offer green dot sights as well.  Although the term “red dot” is still commonly used, due to the variation in color options, the term Pistol Mounted Optic (PMO) is more inclusive.

There are five ways to place a PMO on your pistol:

1. Purchase a pistol with a PMO pre-installed on the slide

2. Purchase a new slide assembly which includes a pre-mounted PMO

3. Install a PMO on a pistol with an existing pre-cut slide

4. Mill an existing slide to create a compatible footprint for installation

5. Install a PMO on the rear-sight dovetail

By far, the easiest and cheapest way is to purchase a pistol with the optic already installed by the manufacturer. Alternatively, you may purchase a separate slide assembly for your pistol which has a factory-mounted PMO. These are available from either from the actual pistol manufacturer (e.g. Sig Sauer) or from an aftermarket company (e.g. Gray Guns). When our CCW permit holders inquire about installing a PMO, I usually suggest obtaining a second factory slide with pre-mounted optic. This gives them the ability to easily switch out slides so they can either carry with a PMO or without.

Otherwise, many newer pistols are sold with pre-cut slides for mounting optics so you can install the optic after purchase. You will simply remove the plate over the cut-out and install your sight with two screws. Mounting the optic is straightforward, but matching the optic footprint to the slide footprint can be complicated.

If you wish to mount a dot on a slide without an existing optic cut, you can send it to a company to "mill" the slide to create the desired footprint. Milling a slide is a permanent modification and can be expensive. Be sure to compare the the price of milling your existing slide vs. purchasing a separate slide with the pre-cut footprint. 

The final option is to use an adapter plate which replaces the rear iron sights. The plate is slid into the dovetail mount. The advantage is that it is cheap, easy, and can be removed if you change your mind. The disadvantage is that the PMO is located higher on the slide, you loose your back-up iron sights, and not mounting is less secure than the other options. I don't recommend the dove-tail mount system. Spend a little money and do it right.

MATCH THE FOOTPRINTS OF THE OPTIC AND THE SLIDE

For those who wish to mount their own optic, it can be tricky. There is no industry-wide standard; both PMO manufacturers AND the pistol manufacturers have come up with a multitude of so-called “footprints.” The footprint of the optic must exactly match the footprint of the slide in order to directly mount the optic to the slide. In a case where you want a specific PMO that will not direct-mount on to your slide, then an adapter plate can be used to change the footprint of your slide. The bottom line is that you must conduct a little research to ensure that the optic you wish to mount will fit on your pistol. One easy method is to search YouTube for both your optic choice and your pistol (e.g. "Can you install a Holosun EPS Carry on a Sig P365?") to see if people have been successful at mating the two footprints.

A footprint consists of three characteristics:

1. Shape

2. Screw pattern

3. Recoil lug pattern

The “Shape” consists of the actual dimensions of the footprint. Dimensions can vary, but an important distinction is the width of the footprint as it correlates to the width of the slide. When first choosing your PMO, determine whether you need a LARGE footprint or SMALL footprint.

• LARGE footprints (1.0 - 1.1 inches in width) are for full-sized (e.g. Glock G17) and compact pistols (e.g. Glock G19)

• SMALL footprints (0.8-0.9 inches in width) are for subcompact pistols (e.g. Glock G43, Sig P365)

The “Screw Pattern” is the location and size of the screws used to secure the optic to the slide or adapter plate. Common footprints use two screws to secure the optic.

The "Recoil Lug Pattern" consists of two or four lugs (male) which extend upward from the slide or adapter plate and fit into lug sockets (female) on the optic itself. The purpose of the lugs is to transfer the energy of recoil to the lugs rather than the mounting screws.

COMMON LARGE FOOTPRINTS

There are numerous available footprints, but I will discuss only a few of the common ones to keep this article on point. Listed below are common LARGE footprints:

RMR:  The Ruggedized Miniature Reflex (RMR) optic is made by Trijicon. The footprint of the optic consists of two screws (white circles in the image below) and two recoil lugs (black circles in the image below). Slides with the RMR footprint are compatible with the following optics: Trijicon RMR and SRO, Holosun 407C, 507C, 507 COMP, and 508T.

Shield RMS:  This footprint was developed by Shield Sights in 1996. The footprint uses two screws and a recoil lug located on the each corner of the footprint. The RMS is considered a LARGE footprint, but has compatibility with the RMSc footprint which is a SMALL footprint (see below). The spacing between the screw and recoil lugs are consistent across the RMS and RMSc, but the overall dimensions are different:

RMS (length x width x height): 42 x 25 x 23 mm

RMSc (length x width x height): 40 x 23 x 22 mm

RMSc optics can fit on RMS slides because they are smaller than the slide cut. The opposite will not be true; a larger RMS optic will not fit on a RMSc slide.

Deltapoint Pro (DPP): The shape, screw pattern and recoil lug pattern are similar to the RMS mount, but the lug diameter of the DPP is 4.2mm vs. 3.9mm for the RMS. This means that when mounting an optic with DPP footprint on a slide with an RMS footprint the recoil lugs are smaller than the female sockets. This prevents the transfer of recoil energy to the lugs. Optics with the DPP footprint include the Leupold Deltapoint Pro, Sig Sauer Romeo X Pro, Vortex Defender ST/XL, and Holosun SCS 320.

Docter/Noblex: This is an older footprint that has two screws and four lugs (at the corners). It will likely become obsolete over time because most optics that use this pattern have been discontinued. A few optics compatible with this footprint are various Burris Fastfire models, the Vortex Viper, and the Vortex Venom.

Glock A-Cut: Glock has partnered with Aimpoint to develop a proprietary mounting system. In the image below, notice that the front of the optic slides under a lip in the slide. To secure the optic, a plate (with the rear iron sights) overlays the back of the optic to secure it from the rear with two screws. Currently, only the Aimpoint COA optic is compatible with this new footprint. Time will tell whether this footprint will catch on.

SIG-LOC: Developed by Sig Sauer for the M17 line of pistols, the LOC (Leveraged Optimized Connection) secures the optic with screws from underneath the slide. This, along with the side accessible battery, allows the optic to sit very low on the slide so that standard size iron sights can be witnessed through the window. Sig Romeo X is the only optic that fits directly on the SIG-LOC footprint.

COMMON SMALL FOOTPRINTS

The number of small footprints is likely low due to the recent evolution of placing PMOs on subcompact pistols.

RMSc: As stated above the RMSc was developed by Shield Sights and is associated with RMS footprint for LARGE footprints. Many recent optics will fit this footprint; however, some optics have an "RMSc-like" footprint, but have some variation that will prevent direct mounting.

Holosun K: The Holosun K optic footprint is a modified version of the RMSc footprint. The screw pattern is identical, but the Holosun K optics will not mount directly to slide cuts that have front and rear recoil lugs. Holosun K optics don't have rear sockets to accept the recoil lugs and the front two sockets are too shallow. As an example, the Glock 43X MOS comes from the factory with an RMSc footprint slide. This footprint has both front and rear recoil lugs that will not allow the mounting of a Holosun K optic directly to the slide.

RMRcc: Recently developed by Trijicon, the RMRcc footprint has a unique design whereby the screws and lugs are integrated (see image below for a milled RMRcc slide). Currently, the only optic that fits this footprint is the Trijicon RMRcc optic.

ADAPTER PLATES

I've focused on the direct mounting of the optic to the slide. However, some pistol manufacturers add complexity by having proprietary footprints which require an adapter plate. An adapter is a thin piece of metal where the bottom has the slide footprint and the top has the optic footprint. Examples of proprietary footprints include the Glock MOS, Walther PDP 2.0 Footprint, Smith & Wesson 2.0 Footprint, H&K VP9 Footprint, and CZ Optics Ready Footprint.

As a example, the Glock MOS (Modular Optic System) offers multiple adapter plates (pictured below, bottom left #01, top left #02, bottom right #03, and top right #04). The commonality is the two large screw holes on the left which mount the plate to the Glock MOS slide. The other screw holes and lugs allow mounting of various optic footprints onto the plate. See the image below for compatibility with various optics (use at your risk). Only the Holosun SCS MOS will directly mount onto an MOS slide.

MOUNTING PROCEDURE

Once you find an optic compatible with your slide, mounting the optic is an easy process if you follow the steps below. Before conducting the procedure, make sure you have the required supplies and equipment. You can acquire the threadlocker, torque wrench, and paint marker for under $50.

THREAD LOCKER: Apply threadlocker to the optic mounting screws (or the adapter plate screws, if required). Use Blue Lock Loctite for this; don't use Red Loctite or Lockset! The latter are extremely difficult to remove if you wish to replace your optic. You may damage the optic or slide trying to separate the two.

TORQUE: Use a torque wrench to obtain the torque (in inch-pounds) as specified by the manufacturer. Under-torquing the optic can lead to it falling off; over-torquing and strip the screws or threads on the slides. First, place the screws in the appropriate holes. Slowly turn the screws, alternating between them every few twists, until they are flush with the optic. Try to move the optic back-and-forth and side-to-side. There shouldn't be any play between the optic and slide or adapter plate. Apply the torque wrench until it clicks indicating that you have reached the setting in inch-pounds on the torque wrench. Don't over tighten!

WITNESS MARKS: Once the sight is installed. Use a white paint marker (e.g. Sharpie brand) to place tics on both the screw (moveable object) and the optic base (immovable object). In the future, if the tics don't line up, it means that the screw has loosened from the initial position. Remove the screws, apply thread locker, and then re-torque the screws to the proper specification. In the image below, you can see a Sig Romeo-2 with witness marks across the screw to the base of the optic. The manufacturer's torque specifications are clearly marked on the base.

ZEROING

Once installed, it's time to zero the optic. A 25-yard zero is common range for pistols. To avoid frustration, I recommend following these three steps:

1. Rough zero with iron sights: This will work if you have suppressor-height sights which can be witnesses through your optic. While aiming at a white wall, line up the iron sights so they are at "equal height-equal light." Using the dials, adjust the red dot so that it lies just on top of the front sight, and centered on the front sight. This will give you a head start for the next step.

2. 5-yard zero: At the range use a 5-yard target for a 25-yard Zero (see image below). You will shoot at the black dot in the top circle. To account for both offset and the closer range, your rounds should impact in the center of the botton circle.

   1. If your rounds are hitting low, you need to raise the point of impact by turning the elevation dial UP

   2. If your rounds are hitting high, you need to lower the point of impact by turning the elevation dial DOWN

   3. If your rounds are hitting to left, you need to move the point of impact by turning the windage dial RIGHT

   4. If your rounds are hitting to RIGHT, you need to move the point of impact by turning the windage dial LEFT

   5. Each click of the dial is usually a 0.5 MOA adjustment (1/2 inch at 100 yards)

3. 25-yard confirmation: If you can find a range that will accommodate 25-yard distances, shoot at a bullseye target to confirm you are sighted in for that distance. You may need to shoot from a bench rest position to get a tighter grouping.

   
   

   

   
   

BATTERIES

Determine if the optic manufacturer recommends a particular brand of battery for their model. Not all battery brands are equal. Avoid off-brand names from Amazon; Duracell is usually a good option. Small changes in battery performance can impact the optic, particularly in reliability. If you have issues with the optic, try a different battery brand before sending the optic in for repair.

Avoid dead batteries (i.e. optic failure) by keeping a fresh battery installed. The current generation of optics are advertised to last 10's of thousands of hours, but don't count on it. I recommend switching out your batteries at least twice a year. Use special dates to help remind you to change the battery (daylight savings time, New Year/4th of July, etc.).