Building an AR-10 Precision Rifle, Part One: The Lower

For Part II: Building an AR-10 Upper Receiver, go here

WE CALL HER LADY LUCK. This tricked out AR-10 in 6mm Creedmoor was inspired by the NRA Designated Marksman Rifle (DMR) Series at Peacemaker National Training Center in Glengary, West Virginia. DMR stands for "designated marksmen rifle," a scoped semi-auto gas gun designed to fill military engagement distances of 300 to 600 yards—or the space between regular infantry fire and longer sniper work.

The Peacemaker matches, and the similar PRS Gas Gun Series, have targets out to 800 yards. We set out to build an Open Class rifle that could confidently engage targets at that distance, and well-beyond. Our only limitations were a caliber of .30 or smaller, and a velocity under 3,200 fps. We settled on the low-recoil and pin-point accurate 6mm Creedmoor round for all the ballistic advantages it brings to the build, coupled with the widening availability of quality 6mm CM factory ammo.

We topped the rifle with a Nikon Black FX1000 6-24x50 for bench testing. The first shots of record with factory-loaded 108-grain Hornady Match ELD measured less than .5-inch.

Lady Luck has gone on to print multiple other sub-MOA groups with the Hornady loads and the 112-grain Barnes Precision Match loads.

Building this tack-driver was a great learning process for everyone involved. We started with a list of high-quality parts all listed below and then teamed up with the guys at Valley Armament in Sayre, PA, who build multiple ARs a week in-shop for clients who demand custom firearms.

The rifle was then brought to Nick Kesselring of FSG Custom, who took the idea of a long-range precision to a whole new level with WWII military aircraft themed engravings and Cerakoting.

The lower alone racked up seven hours of machine time, etching in the B52-skin-like rivets and panels, and our lucky lady riding a bomb.

The end result is truly a work of art.

The AR-10/LR-308 platform is not standardized like the smaller 5.56 AR-15 platform. So building an AR-10 requires some special considerations. Most AR-15 small parts work in an AR-10 lower, except:

Those lower parts are AR-10 specific. AR-15 parts will not work. Additionally, all AR-10 uppers and lowers are not standardized.

There are two main “pattern series” of receivers: Armalite and DPMS. If you are shopping for a lower, we suggest the DPMS pattern as there is a wider selection of parts available.

This build used a Bushmaster .308 DPMS-patterned upper and lower receiver. The DPMS pattern is technically a LR-308, opposed to the Armalite ATR-10, but for simplicity sake we're calling all builds that take a .308-length cartridge an AR-10.

Something else to keep in mind: different AR-10 lowers accept different style magazines. Though they have mostly standardized around SR-pattern mags.

The Magpul PMAG 10 LR/SR GEN M3 magazines we used are a popular example. We recommend you ensure your lower takes SR mags for the simple reasons of cost and availability.

To start, lay out all your parts on your work surface. It doesn’t just make a cool photo, but you’ll quickly see if you’re missing anything.

Now is also the time to dry fit the grip to the lower receiver and change backstraps as needed to get the right fit with no gaps.

A grip with interchangeable backstraps, like the Magpul MIAD series, is recommended for AR-10 projects like this. Most standard AR-15 grips leave an unsightly gap between grip and AR-10 lower. The MIAD does not.

A magnetic parts tray is also handy. So, is a Carpet Kit from 2A Armament as it's very easy to lose springs and plungers.

You're not installing the grip yet, just making sure it's all ready to go. We'll get to that later.

When assembling a stripped lower, there are several ways to start, but the guys at Valley Armament begin with the mag catch/mag release.

We used Odin Works XMRs on all three rifle builds in this project, but the process is the same for most catch/release installs.

There are three basic parts: the magazine catch bar, catch spring, and release button.

On mil-spec parts, the catch bar is hard-welded to a post that screws into the button. On others, the post is separate and the bar is screwed to the post and the button.

Whatever the case, line up the bar on the outside of the left side of the lower, with the post running through the receiver. Then, slide on the spring and button from the right side in.

Next, take a dowel or push pin and depress the spring and button into the lower so the catch bar stands proud, and screw the post into to the button.

When the assembly comes together so that the button and catch are flush with the side of the lower, stop threading and check to see how it works with an unloaded magazine. If it holds the magazine firm and readily releases it when the button is depressed, it's good to go. Some minor adjusting one way or the other may be necessary here to get it just right.

Next, install the bolt stop. Start the roll pin with a few taps, then drop the spring and plunger in the detent hole just under where the paddle sits, above the magazine catch bar.

Put some pressure on the paddle, depressing the spring and plunger, and capture it in place with a small hex wrench. Now, carefully hammer the roll pin in place.

Get a takedown pin pivot tool and slide it through the eyes where the front takedown pin goes. Rotate the tool until the holes line up with the detent spot, and drop in the spring and the plunger.

With your small Allen key, depress plunger and spring and roll the tool to capture them in place. Now carefully slide the takedown pin in place of the tool.

For a video demonstration of this, go here

For all our R365 AR builds we went with Timney Triggers. This 6mm Creedmoor match rifle got the very best, a Calvin Elite single-stage set at a pleasing 1.5 pounds with zero creep.

Installing one is as simple as dropping it in, then lining up the holes in the trigger with the holes running through the lower, and pinning it in place.

It should be noted that mil-spec triggers install in two pieces—hammer and trigger—but the performance advantage you get from a single-unit drop-in like the Timney make them well worth the cost. For all but the least expensive budget builds, get a drop-in trigger.

Timney triggers come with locking pins, which is a nice touch—and means the pins will never walk themselves out. Don't tighten the pins all the way down until the selector is installed (next step).

Like so many components of an AR, the selector is held in place by plunger and spring. The plunger and spring, in this case, is held in place by the pistols grip and the large bolt that secures it to the lower.

To install everything the selector switch and the grip, simply run the selector through the receiver and attach both levers, if it’s an ambidextrous switch.

Next, drop that plunger into the small hole in the lower under the switch, and then place the spring in the small hole on top of the grip.

Connect the grip to the lower, marrying the spring and plunger. When everything is lined up, tighten the grip down with the internal screw inside the grip. You'll need a good length screwdriver for this. Keep your eyes on that spring as your tightening the grip down. If it's not lined up absolutely perfectly (and sometimes even it it is) that spring will bend and pop out the side. You don't have to pull it all apart if you go slow. When the spring first begins to pop out, get a small tool or punch and just push it back into place and continue to tighten down the grip.

Then, don't forget to tighten up the trigger pins.

Now, run a safety check, ensuring the trigger can't be pulled when the safety is engaged.

The buffer tube, rear take down pin, and buffer assembly all work together, so they’re installed together in this single step.

For this AR-10 build we went with an A5-length buffer tube to accommodate a rifle-length buffer and our fixed stock.

To get started, first put your end plate and castle nut on the buffer tube and screw it loosely into the lower making sure you have about an inch of space between the end nut and lower.

Next, slot the rear takedown pin through the receiver.

Install the takedown pin detent and spring in the back of the lower much the way you installed the front takedown pin. It will fit in that inch-or-so space you left between the end plate and lower.

Once installed, compress the spring with the end plate and hold in place. Again, this is one of those springs that can pop out of the side as you tighten down the end plate. Be careful not to pinch the spring so as to deform it and push it back in if it starts to make an escape.

Now, drop the buffer spring and plunger while holding the end plate and screw down the buffer tube until the key-notch captures the buffer plunger inside the lower.

Everything should now be lined up and you can torque down the castle nut to 35- to 40-foot pounds—lean lighter for AR-10 or rifle-length builds, heavier for carbines.

Mil-spec rifles required a “staked” castle nut—a hammer and punch or chisel is used to strike the nut in place, ensuring it can’t walk in the threads. We did not do this on any of our builds.

Fixed buttstocks like this PRS GEN3 Magpul are installed a little differently than the more common collapsible designs, but really is as easy as sliding it onto the buffer tube, and tightening it down.

For the buffer assembly, we opted for the very best. A match rifle cannot fail. High pressure rounds like our 6mm Creedmoor, or 6.5 Creedmoor, have had known cycling issues in gas guns.

Our solution to this was the Hydraulic KynShot AR-10 buffer and a JP Tuned & Polish Extra Power spring—with an Odin Works Rifle XL gas system in the upper. It worked flawlessly. These parts install like any stock AR buffer assembly.

For the final step, depress the buffer detent, and feed spring and buffer into the buffer tube.

With that, your precision AR-10 lower is built.