Machining a Glock slide for a Trijicon RMR cut

Allowing the shooter to only focus on one plane when shooting, red dot sights, long established on carbines, are gaining in popularity on pistols. A Glock equipped with a Trijicon RMR red dot sight seems to be the most commonly encountered configuration. The combination has become so prevalent, multiple manufacturers offer replacement Glock slide precut for the red dot sight. I decided it was time to jump on the bandwagon.

I did some research on the net prior to starting my Glock RMR project. The biggest decision seems to be where to mount the optic. The optic can be mounted in front of the existing sight, flush with the rear of the slide, or set in from the back a little with a new sight in place. I decided to install the RMR about .200″ from the rear of the slide, with the rear sight in front of the optic. By having a ledge of material behind the optic, it is mechanically locked into place.

Placing the optic behind the rear sight has a number of advantages, it allows easier modification of holsters, protects the lens of the optic from brass and residue, and provides a wider field of view through the optic.

Trijicon will provide a drawing of the base of an RMR if you email and ask for one. They do not provide instructions for milling the slide.

On the mounts Trijicon makes for the RMR, the optic is retained to the base by two .1539″ diameter studs at the front of the optic, and two 6-32 screws in the rear (see image below). Cutting the two studs would be difficult on a manual mill, however, since the optic is set into the slide, and since the front of the optic has a 3.0″ radius, the optic is physically prevented from moving sideways. Keep in mind, the mounts Trijicon makes, only have the optic secured in those four places, dropping the optic into the slide allows for greater support.

A factory Trijicon RMR mounting plate (above). This one adapts the RMR to a weaver rail. Note the two studs (bosses) on the top of the plate.

I’ve seen a few different depths of cut discussed for the RMR inlets, .121″ deep seems to be the most common. Examining the bottom of the slide, you can note cuts underneath to help determine how deep you can cut into the slide. I decided to cut my slide .121″ deep.

The gun used in this post is a Police Trade-in Generation 3 Glock 22 I bought used. It was in pretty sad shape so I figured it would be the perfect test gun. It cost $300 shipped, that way if I accidentally destroyed the slide it would hurt less. Note “Fulton County Ga. Police” is engraved on the right side. Based on its internal wear, I assume this was either a pool gun or belonged to someone who worked at the range and shot all the time. It is one of the most worn Glocks I have encountered.

Set up is key to machining the slide for the RMR. On a manual mill the radius on the front of the sight is the most difficult cut to make. I made mine on a rotary table. After that, the rest of the steps are fairly straight forward.

I ordered the following items from Brownells for this project:

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Any modifications made to a firearm should be made by a licensed gunsmith. Failure to do so may void warranties and result in an unsafe firearm and may cause injury or death.

Modifications to a firearm may result in personal injury or death, cause the firearm to not function properly, or malfunction, and cause the firearm to become unsafe.

I begin by bolting the rotary table down to the mill. A coaxial indicator is used to center the rotary table under the mill. I zero the digital readout (DRO).

The front radius of the RMR sight is 3.000″ +/- .010″. Since I will be using a 3/8 (.375″) end mill to cut the arc, I subtract the end mill’s radius (.188″) from the radius of the arc (3.000″), .3.000″-.188″=2.812″. Offsetting the x-axis 2.812 inches will allow the end mill to cut in 3.000″ arc.

I make a light cut on a piece of scrap and test fit it to the sight. It fits perfectly.

I mark the location of the front edge of the RMR cut on the slide. The slide held on a pair of 1-2-3 blocks which are secured with clamps. The mills y-axis is moved back half the width of the slide (1.005″/2=.503″) and a .0005″ test indicator is used to align the slide. This will ensure the arc is centered on the slide.

The y-axis is returned to 0. I make a light pass on the slide to ensure the cut is correct.

A quick check with the RMR shows the radius is perfect.

I continue milling the radius until the cut is .121″ deep.

The slide is set up on a pair of parallels in the mill. An edge finder is used to locate the center line of the slide, as well as the front edge of the cut. The optic is 1.780″ long.

I start with a series of light passes to make sure everything fits.

Another quick check with the RMR. It snaps right in!

I mill the entire recess until it is .121″ deep and test fit the RMR again. It snaps right in.

The optic is held in place by two 6-32 screws. I located the holes with a spotting drill (above).

A tap hole is drilled for each screw.

A 6-32 tap coated in tapping paste is secured in a tap handle held perpendicular by a tap guide in the mill. I used a cheap high-speed steel tap set for this operation (I paid $8 shipped for three taps), that was a mistake. The steel on a Glock slide is tough and chewy, it would have been better to use a higher quality tap. I was worried I’d break the tap, fortunately, I didn’t. I would recommend buying a high quality, high-speed steel taper and plug tap from an industrial supply house like MSC for this operation.

The rear sight on a Glock requires a 74 degree dovetail cut. The cut is 2mm deep, which converts to approximately .079″. Note the slight ledge on the back of the Ameriglo rear sight in the photo above.

Prior to making the rear sight cut, I locate the front edge of the RMR cut. The centerline of the rear sight will be .250″ forward of the RMR cut.

Since dovetail cutters are fragile and expensive, I make a cut with a 3/16″ solid carbide end mill to remove the majority of the material from the dovetail slot.

Unless you like spending money on cutters, go slow. I ran my mill at the lowest spindle speed and fed the table at the slowest rate. The cutter was lubricated with Do-Drill cutting oil.

The finished dovetail looks good.

The rear sight is oversized so the dovetail needs to be opened up slightly. I used a 60 degree bent file.

The bottom of the RMR is open (above, left). Note the o-ring around the perimeter of the base, this seals the unit when it is secured to a mount. Since some of the o-ring on the bottom of the sight isn’t compressed against the Glock slide cut, a sealing plate (above, right) is used to ensure the optic is protected from the elements.