Gen 3 UHP15, UHP15A and UPUR-1 assembly notes.
A Gen 3 UHP15 or UHP15A ambi lower builds with a standard AR-15 lower parts kit, and the UPUR-1 upper takes most aftermarket handguards with the barrel nut torqued to no more than 70 ft-lb. Two things are already in either receiver when it arrives: a threaded bolt catch pin (remove it with a 1/16 in. Allen wrench before you install the bolt catch) and a lower-to-upper tensioning screw (adjust it with a 1/8 in. Allen wrench). The ambi lower differs in one part that matters: the UHP15A comes with an extended bolt catch detent in a bag zip-tied to the trigger guard, and it must replace the detent in your kit or the right-side bolt release will not work. The UHP15 uses the kit's detent. This page is both in-box sheets, with the ambi and non-ambi notes in their own sections.
Parts kit, magazines and finish.
Lower parts kit
A standard AR-15 lower parts kit. It includes the correct takedown and pivot pins and the correct bolt catch for the UHP15 and UHP15A.
Magazines
AR-15 and M16 style magazines only.
Finish
Type III Class 2 hard anodize. New anodizing can carry a slight film; a light oil such as Rem Oil takes it off.
Both lowers: what is already installed.
The Gen 3 UHP15 and the Gen 3 UHP15A share these three notes. The sheet in the box is different for each lower; the two sections after this one say what is specific to each.
- Bolt catch pin. The threaded bolt catch pin is provided and pre-installed in its hole. Remove it with a 1/16 in. Allen wrench before installing the bolt catch assembly, then thread it back in through the bolt catch.
- Tensioning screw. A lower-to-upper tensioning screw is pre-installed in the lower. The nylon- or brass-tipped screw adjusts with a 1/8 in. Allen wrench to take the play out of the upper-to-lower fit.
- Safety selector. Both lowers are compatible with the Battle Arms Development 45-degree safety selector. The fire marking is a quarter-radius mark that reads correctly at either the 90-degree or the 135-degree position, depending on the selector you install.
UHP15A ambi lower only.
- The ambidextrous magazine release lever works with the standard mag catch arm supplied in a lower parts kit; nothing extra is needed there.
- Everything else on the UHP15A is the standard AR-15 kit: takedown and pivot pins, bolt catch, trigger group, buffer assembly.
This applies to every ambi lower and set: the Gen 3 UHP-15A lower, the Gen 3 UHP15A combo set, the A.B.S. ambi builder set and the TANO ambi sets.
UHP15 (non-ambi) lower only.
- The Gen 3 UHP15 builds with the bolt catch detent from the lower parts kit. The extended detent is a UHP15A part; there is none in the UHP15 box and none is needed.
- The Gen 3 UHP15 and UPUR-1 are compatible with the Tactical Link Enhanced Battery Assist Lever.
This applies to the Gen 3 UHP-15 lower and the AR-15 billet combo set.
The UPUR-1 upper.
- Designed to be compatible with most aftermarket handguards.
- Use a lubricant when inserting the barrel into the upper, and grease or anti-seize on the barrel nut.
- It is fine to warm the upper with a hair dryer to ease the barrel in.
- Do not exceed 70 ft-lb on the barrel nut, or the barrel nut maker's own limit if it is lower. The CMT barrel nut's values are in the handguard install guide.
- The forward assist roll pin installs from the bottom side of the receiver.
- Do not run the bolt carrier group without lubrication.
Compatibility
The Gen 3 UHP15, UHP15A and UPUR-1 are designed to be compatible with other receivers machined to mil-spec dimensions. Allowable tolerances stack up between manufacturers' components, so some minor fitting may be required.
Frequently asked questions.
Does the UHP15 take a standard AR-15 lower parts kit?
Yes. The standard AR-15 kit includes the correct takedown and pivot pins and the correct bolt catch. The only CMT-specific parts are the threaded bolt catch pin and the tensioning screw, which arrive installed in the lower, and on the UHP15A the extended bolt catch detent in the bag on the trigger guard.
What is the threaded pin already in my lower?
It is the bolt catch pin. CMT threads it instead of pressing a roll pin, so the bolt catch goes in without a punch and without marking the receiver. Back it out with a 1/16 in. Allen wrench, fit the bolt catch, spring and plunger, then run the pin back in.
Is the build different for the UHP15A ambi lower and the UHP15?
Yes, in one part. Both take a standard AR-15 lower parts kit and both arrive with the threaded bolt catch pin and the tensioning screw installed. The UHP15A ambi lower must be built with the extended bolt catch detent from the bag on its trigger guard, for the right-side bolt release paddle; the UHP15 is built with the detent in the kit. The UHP15 is also the one listed as compatible with the Tactical Link Enhanced Battery Assist Lever.
Why does the UHP15A need the extended bolt catch detent?
The right-side bolt release paddle on the ambi lower acts through the bolt catch detent, and the standard detent in a parts kit is too short to reach it. The longer detent is in the bag zip-tied to the trigger guard; use it in place of the kit's detent.
How do I take the play out of the upper-to-lower fit?
Turn the pre-installed tensioning screw with a 1/8 in. Allen wrench until the upper sits the way you want it. The tip is nylon or brass, so it bears on the upper without marking it.
How tight does the barrel nut go on a UPUR-1?
No more than 70 ft-lb, and no more than the barrel nut maker's own limit if that is lower. Lubricate the barrel where it enters the upper and grease the nut threads. With a CMT rail and the CMT barrel nut, the values are 50 ft-lb, back off a half turn, then 50 to 70 ft-lb.
Which magazines fit?
AR-15 and M16 style magazines only. The flared, broached magwell drops a mil-spec magazine cleanly; the UHP15 and UHP15A are not cut for pistol-caliber or .308 magazines, which have their own CMT lowers.
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.