In building our NRA DMR / PRS Gas Gun Series AR-10, we started with a stripped DPMS-patterned Bushmaster .308 lower and upper.
For backstory on this build, or to see the lower built, visit our AR-10 lower builder’s guide.
AR-10 Upper Considerations
Uppers can be more difficult to assemble than lowers, as they require some special tools. There’s also more risk of injury or worse if an upper is poorly assembled. The safest bet is to buy a pre-assembled upper, or seek the help of an accomplished builder.
The AR-10/LR-308 platform is not standardized like the AR-15. When assembling an AR-10 upper, most AR-15 parts do not work. You’ll need a caliber-specific:
- Barrel
- Bolt Carrier Group (BCG)
- Muzzle device
Plus an AR-10 specific forend and charging handle. Really, only the forward assist is the same between an AR-15 upper and an AR-10 upper.
Depending on the pattern of your upper receiver, you’ll also need the correct barrel nut to ensure the handguard sits flush with the top rail of the receiver. DPMS-pattern uppers have two sizes, high and low. Check with the manufacturer to see which size you have, and order the barrel nut/handguard accordingly. Our Bushmaster .308 upper was DPMS low.
Getting Started
- Orientation. For our purposes, “left” and “right” refer to sides of the rifle from the shooters perspective when the rifle is pointed forward.
- Work Surface. Make sure you have a clean well-lit area to work. Try our Yoga Mat trick. A drop cloth under your work area saves time hunting for dropped roll pins, too.
- Tools. It doesn’t take tons of tools to build an AR, but these tools are worth the investment.
- Safety. Eye protection is highly recommended. Springs especially like to fly.
Parts List
| Upper Receiver | Bushmaster .308 Upper Receiver Assembly |
| Forward Assist | Radian Forward Assist Aluminum Black Anodized |
| Barrel | Odin Works 6mm Creedmoor 22-inch, Rifle +2 |
| Barrel Nut | Odin Works AR10 Armalite Foreend Adapter & Barrel Nut |
| Gas Block | Odin Works Tunable Low Profile Gas Block |
| Gas Tube | Odin Works XL Rifle (Rifle +2) Gas Tube |
| Muzzle Device | Odin Works Atlas 6.5 Compensator |
| Bolt Carrier Group | Odin Works Black Nitride BCG |
| Charging Handle | Radian Raptor Ambidextrous |
| Forends | Odin Works O2 Lite AR10 M-LOK |
| Optic Mount | Nikon BLACK Precision Mount MSR |
| Optic | Nikon BLACK FX1000 6-24x50SF Illuminated FX-MRAD |
The Build
Install the Forward Assist
If your upper doesn’t have a forward assist installed, it’s best to do this first. If your upper doesn’t have a port for a forward assist, you can skip this step.
In this step, and most other steps involved in putting together an upper, a bench vise and dedicated vise block is highly recommended.
To start, get your upper locked down in a vise and block, then start the forward assist roll pin with a pair of vise grips or hollow punch. Insert the forward assist and spring, with the “pawl” or curved edge facing in. The flat side is where the pin rides.
Lap The Receiver
Lapping an upper receiver takes the rough-finished coated metal around the barrel port and makes it true and as flat a possible. You use a gritty clay-like compound and a rotating tool chucked into a drill to sand away a small amount of metal material.
Wheeler makes a kit for AR-15s and AR-10s that make this job easy using a power drill. This is not necessary but recommended for a precision build. You can watch our video demonstration here as the process is the same for an AR-15 as it is for an AR-10.
Cover the lapping bar in lapping compound, and spin it in the barrel slot with a drill on low speed for 10 to 20 seconds. The goal is to take away just enough metal coating until you get down to the shiny aluminum, true, receiver face.
Install the Barrel
The barrel is where rifle accuracy is made or lost. We opted for the very best we could find for this match build: the Odin Works 6mm Creedmoor 22-incher.
Odin contours, finishes, and hand-laps Bergara blanks in their Oregon shop. This high-quality tube is fitted with their property Rifle XL gas length system, which is rifle length plus 2 inches.
The increased dwell time helps tame high-pressure rounds like the 6mm CM. Combined with our KynShot/JP buffer system, it worked flawlessly.
It also has a polished M4 feed ramp. Some 6mm and 6.5mm AR builds have suffered from poor feeding. This was not the case with our Odin setup.
To install a barrel, first clean the upper making sure no lapping compound was left over if you chose to do the previous step.
Take a brass brush and clean out the threads to ensure proper fitment. Slide the barrel into the upper, matching the barrel key to the notch in the receiver.
Screw on the barrel nut, then torque it to 30-foot pounds, or the recommended spec from your upper/barrel manufacturer. Getting the proper torque may take some tools. If your handguard attaches directly to the barrel nut, now is the time to confirm the handguard lines up with the upper.
Install Gas System
The Odin 6mm barrel comes with their Tunable Low Profile gas block and Rifle XL gas tube. As mentioned, high-pressure rounds can be finicky when it comes to cycling, but Odin’s dedicated gas system for these competition rounds works well. We ran ours with the gas block all the way open.
To install, start the roll pin in the topmost gas block hole with a pair of vise grips or hollow punch. Line up the hole in the gas tube with gas block, ensure the curve of the tube runs up.
When aligned, drive the roll pin all the way through, slide gas block over the barrel, and slot the tube in the upper. If there’s a little back-and-forth play in the gas tube at the upper, it’s aligned correctly.
Install the Handguard
As mentioned, there are two main handguard sizes for DPMS-patterned AR-10s, high and low. We originally ordered a high forend adaptor by mistake, so the handguard stands a bit high relation to the rail on the upper receiver, as you can see in these photos. We corrected this later.
Getting the correct nut is easy enough, but it did delay our build project. So, we learned it always pays to have your ducks in a row before you break out the tools.
Nearly all handguards attach the same way—an adaptor slots over the barrel nut, or a dedicated barrel nut is included, and then the handguard screws into that.
Install a Muzzle Device
Because the heart of this build was an Odin Works barrel and gas system, it only made sense to run their ATLAS Compensator.
The three-piece modular design is timed via the titanium sleeve, so no shims or crush washers are needed.
If you buy the 6mm Creedmoor barrel from Odin, this comp is offered at a reduced price.
For a self-timing brake, like the Odin, simply thread it on, line it up, and lock it down. If your brake requires a crush washer, first make sure the cupped side of the washer faces out.
Then, hand tighten the brake until it stops. Get a wrench and turn the brake—crushing the washer—until it’s lined up straight and level with the gas block.
Assemble the Bolt Carrier Group
To further match the Odin internals in this build, we used their AR Black Nitride BCG—the bolt matched to the barrel, something we recommend regardless of your barrel maker. We pair this with a charging handle from Radian, and snapped the whole thing into place.
Assembling a bolt carrier group (BCG) can be a bit of a puzzle the first time you do it. To make it easier, take a look at the video above detailing an AR-15 BCG assembly—there are no significant differences between an AR-15 BCG and an AR-10 BCG, so it’s the same process for both.
A bolt carrier group is made up of six parts: The carrier, gas key, bolt assembly, firing pin, bolt cam pin, and firing pin retaining pin.
In nearly all cases, the gas key comes locked down to the carrier. Keep it that way. Many of them will be staked so the screws won’t back out under recoil, so removing the gas key is impossible anyway (check periodically after firing to make sure the gas key is still tight).
• Assemble the BCG by first determining which way the cam pin fits in the bolt assembly. It will slide in nicely one way, and not fit the other. The cam pin has a hole in it through which the firing pin is inserted. Keep that in mind.
• Put the bolt assembly into the carrier so the hole for the cam pin is facing up and the extractor is on the right side.
• Insert the cam pin so the firing pin can be inserted through the bolt and the pin and then turn it 90 degrees to lock it into its channel on the carrier. The bolt can now move back and forth in the carrier.
• Then, from the back of the carrier, drop in the firing pin so that it goes through the hole in the cam pin.
• Secure the firing pin in place with the retaining cotter pin. This can usually be done with your hands.
• Pull on the bolt face, and it should move back and forth and rotate within the BCG.
The process is the same for an AR-15 BCG, so you can use the video above as a guide too.
Insert The Charging Handle and BCG
To install the BCG and charging handle in the receiver, make sure the bolt is fully extended from the carrier.
Then, seat the charging handle in the upper receiver, and slide the BCG in so that the gas key is lined up in the groove on the bottom of the charging handle.
Everything should fall into place. Nothing will be under tension until the buffer spring is released when the upper and lower are assembled.
Install an Optic
For optics we wanted big, clear glass that didn’t break the bank. The Nikon FX1000 6-24×50 scope with the Nikon Black mounts were a natural choice and haven’t let us down yet. For complete instructions for mounting a rifle scope the right way, go here.
The “Lady Luck” DMR Rifle
After Nick at FSG Customs did his Cerakote and laser etching magic, the rifle was kitted out with the following accessories:
| Engraving & Cerakote | FSG Customs |
| Accessories | Magpul Bipod for M-LOK |
| Magpul MS4 Dual QD Sling GEN2 | |
| Magpul M-LOK Tripod Adapter | |
| Magpul PMAG 10 LR/SR GEN M3 |
Accuracy, reliability, looks. Lady Luck has it all. But the real test will be next summer, when we set her on the firing line at NRA DMR Series at Peacemaker. Then it’ll all be up to the shooter.