How to install a CMT handguard.
Every CMT handguard installs the same way, AR-15 or AR-10, UHPR, HDM, DRT, TANO or ARCA: torque the CMT barrel nut to 50 ft-lb, loosen it one-half turn, torque it to 50 to 70 ft-lb, fit the gas block and gas tube, slide the rail onto the nut, align it with a scope ring laid across the upper and the rail, then clamp it with the two retention clamps at 55 in-lb. The nut is never timed. This page is the install card that ships in the box.
The numbers.
| Fastener | Torque | Tool |
|---|---|---|
| CMT barrel nut, first pass | 50 ft-lb, then loosen one-half turn | CMT barrel nut wrench and a torque wrench |
| CMT barrel nut, final | 50 to 70 ft-lb | CMT barrel nut wrench and a torque wrench |
| Rail retention clamp screws (two 8-36 socket head cap screws) | 55 in-lb, with blue Loctite on the clamps | 9/64 in. Allen wrench and an inch-pound torque wrench |
What you need.
In the box
The rail, the CMT barrel nut, two rail retention clamps and two 8-36 socket head cap screws.
Tools
The CMT barrel nut wrench, a foot-pound torque wrench, an inch-pound torque wrench, a 9/64 in. Allen wrench, blue Loctite, a light lubricant or anti-seize, and an upper receiver vise block.
For alignment
A scope ring or any rail-mounted device that spans the upper receiver rail and the handguard rail.
Prepping the rail.
- Remove the two 8-36 socket head cap screws from the rail retention clamps with a 9/64 in. Allen wrench, then remove the retention clamps from the rail.
- Pull the barrel nut out of the rail. The nut is a close fit in the rail bore, so it may be necessary to thread the upper receiver into the barrel nut two or three turns, with no barrel in the upper, and use the upper to pull the nut out.
Installation, step by step.
- If the upper currently wears a different rail system, remove the existing rail, gas block and barrel nut.
- Clean the threads on the upper receiver and apply a minimal amount of lubricant to them to prevent galling.
- Test fit the barrel nut on the upper receiver threads, then remove it.
- Test fit the rail to the upper to make sure the anti-rotation tabs on the rail fit your upper without modification. Some billet uppers are not machined to mil-spec dimensions here; if yours is not, the alignment tabs can be removed and the rail timed to the upper with a scope ring until the barrel nut clamps are torqued. (Only the AR-15 rails have anti-rotation tabs.)
- Insert the barrel into the upper properly.
- Apply a minimal amount of lubricant to the upper threads and to the outer face of the barrel extension where the barrel nut tightens onto it.
- Using only the provided CMT barrel nut wrench and a torque wrench, torque the barrel nut to 50 ft-lb, loosen it one-half turn, and final torque it to 50 to 70 ft-lb.
- Install your gas block and gas tube.
- Slide the handguard onto the barrel nut slowly, making sure the timing tabs are oriented correctly to locate on the upper. (AR-15 rails only.)
- Install one rail retention clamp into its slot, checking by eye from the opposite side that the slot in the barrel nut lines up with the key in the clamp. Apply blue Loctite to the retention clamp so it cannot loosen.
- Once the retention key alignment is confirmed, install the second retention clamp, only starting the screws.
- Lay a scope ring or other rail-mounted device across the Picatinny rail from the upper receiver onto the handguard, so the two rails are aligned before the clamp screws are tightened.
- Tighten the retention clamp screws to 55 in-lb.
Fit and compatibility.
CMT handguards are designed for upper receivers machined to mil-spec dimensions. Allowable tolerances stack up between different manufacturers' components, so some minor fitting may be required. AR-10 rails come in two heights, commonly called high profile and low profile; the HP or LP in a CMT part number says which upper receiver height the rail fits, so match it to your upper before ordering. The AR-10 rails do not carry anti-rotation tabs.
Rails this guide covers
Every CMT free-float handguard: UHPR MOD 2 HDX, UHPR MOD 3 quad and MOD 4 hybrid, UHPR MOD 2-HP and ARCA-HP 308 rails for the AR-10, HDM, HDM-A2 ARCA, DRT, DRT-A2 ARCA, TANO, the micro rails and everything on the ARCA shelf. Blemished rails install the same way.
Frequently asked questions.
What torque does the CMT barrel nut take?
Torque it to 50 ft-lb with the CMT barrel nut wrench, loosen it one-half turn, then final torque it to 50 to 70 ft-lb. Lubricate the upper receiver threads and the face of the barrel extension lightly first so the threads do not gall.
What torque do the rail clamp screws take?
55 in-lb on each of the two 8-36 socket head cap screws, with blue Loctite on the retention clamps. Align the handguard rail to the upper receiver rail with a scope ring before you tighten them.
Do I have to time the CMT barrel nut to the gas tube?
No. The CMT barrel nut is not timed and has no shim stack. Torque it to the values above, confirm the gas tube passes through it without touching, and the rail locates on the nut and the upper.
The anti-rotation tabs do not fit my billet upper. What now?
Some billet uppers are not cut to the mil-spec dimension the tabs need. Remove the tabs, then time the rail to the upper with a scope ring laid across both rails while you torque the retention clamps. Only the AR-15 rails have the tabs; the AR-10 rails are aligned the same way with a scope ring.
Which AR-10 rail height do I need, HP or LP?
AR-10 uppers come in high-profile and low-profile heights, and a rail only sits flush on the one it was cut for. The HP or LP in the CMT part number is the upper receiver height the rail fits. CMT's current 308 rails are the HP versions; check your upper before ordering.
Do I need a gunsmith to install a rail?
The card says a competent gunsmith or armorer should do it. If you own a vise block, the CMT barrel nut wrench and both torque wrenches, and you follow the steps above, the job is straightforward. If you do not, have a qualified gunsmith do it, and have one check the safety and function of the firearm before use either way.
Notice.
No liability is expressed or implied for damage, injury or death resulting from the improper installation or use/misuse of Cross Machine Tool products.
Like climbing mountains, racing motorcycles or jumping from perfectly good airplanes, ownership and handling of firearms carries with it certain unavoidable risks. If you are not willing to accept the responsibility for your own actions, then firearm ownership is not for you.
- The use of non-compatible custom parts or modification of these parts will void your warranty.
- Returns will not be accepted on any non-defective part that has been installed or modified.
- Before cleaning or performing any work on your firearm, make sure that it is clear. Remove the magazine and visually inspect the chamber to ensure the firearm is completely unloaded.
- It is strongly recommended that a qualified gunsmith check the safety and function of your firearm before use.
- It is your responsibility to ensure that the firearm you are building is legal to build and possess in your city and state.
- Must be 21 years old to purchase this product.
Compatibility with other components
Due to the abundance of aftermarket components available and introduced each year, it is impossible for us to provide compatibility information for all components with our products. Some simple measurements, visual examination and some common sense can save a lot of time. Ultimately, it is your responsibility to verify that the parts you intend to use are compatible.
Questions or a warranty issue: email support@cmttac.com. You will receive a response within one business day.