Building a basic AR-15 isn’t much more difficult than assembling IKEA furniture. You’ll need some hand tools, patience, and will have to refer to instructions the first few times, but it’s relatively tough to screw up if you’re mechanically proficient. Most parts fit together interchangeably, and if you run into a problem, it’s easy to find recommended solutions. The AR-10, however, is a different beast.
Unlike Eugene Stoner’s 5.56x45mm golden child, the AR-10 wasn’t widely adopted for military service and therefore wasn’t standardized to the same extent as the AR-15 and M16. As a result, attempting to build — or even modify — an AR-10 sometimes feels like navigating a minefield of branching paths and incompatibilities. Do you have an Armalite AR-10 or DPMS (a.k.a. LR-308) style receiver set? If you have a DPMS, is it Gen 1 or Gen 2? Are your BCG and barrel extension Armalite style, DPMS style, or an oddball like the Rock River Arms LAR-8? Even factors such as Picatinny rail height and barrel nut thread pitch aren’t consistent across all AR-10s.
Due to the more complex nature of assembling an AR-10 from assorted parts, many of us start our large-frame AR projects with a complete rifle or builder’s kit. This ensures that core components like the receivers, handguard, barrel, and BCG will work together, but even then, simply adding a suppressor can be enough to throw a wrench into the system. I learned this firsthand with my Stag Arms Pursuit AR-10, an 18-inch rifle chambered in 6.5 Creedmoor.
Armalite, DPMS, or Something in Between?
At first glance, you might think the Stag 10 Pursuit is an Armalite-pattern rifle inside and out, since its receivers have the characteristic angular cut behind the rear takedown pin. However, like its sibling the Aero M5 — which has a rounded receiver cut — it uses all DPMS Gen 1 style components. While certainly confusing, this is actually beneficial, since most aftermarket AR10 parts are designed for DPMS specs.
Going Suppressed
After running a few hundred rounds of Norma Golden Target 143gr 6.5 Creedmoor through the rifle in unmodified form, I decided to swap out the original VG6 Gamma muzzle brake for a Daniel Defense DDWave direct-thread 3D-printed suppressor. This is where the trouble started.
The increase in gas pressure caused frequent failures to feed with the bolt overriding rounds and smashing the brass. Even when the rifle fed correctly, chambered rounds showed substantial tip deformation from impacting the feed ramps at high speed. On a precision-oriented rifle, this can lead to unacceptable deviations in accuracy. The recoil impulse of each shot felt harsher, and spent brass was violently hurled out of the ejection port.
I reached out to Stag Arms customer support about this issue, and they agreed with my conclusion that the rifle was likely overgassed. While adding an adjustable gas block might’ve been the quick fix (or not — more on that later) I decided to start with another one of the Stag rep’s recommendations: change the buffer system.
Buffer Swap
The Stag Pursuit rifle came with an AR-15 carbine length buffer system, as opposed to the longer “intermediate length” system used on some AR-10s (as well as AR-15s using VLTOR’s A5 system or BCM’s MK2 system). If you’re not sure which you have, check the internal depth of the tube — AR-15 carbine is roughly 7 inches deep, while intermediate is 7.75 inches deep.
If you have an AR-15 carbine tube on your AR-10, it must be used with a 2.5-inch shorty .308 buffer, since an AR-10 bolt carrier is longer than an AR-15 carrier. If you have an intermediate or A5 length tube, you can use a standard 3.25-inch-long AR-15 carbine buffer.
Some users claim that a longer buffer system softens recoil slightly, while others perceive no difference. One notable advantage to an AR-10 with an intermediate length tube is the aforementioned compatibility with regular AR-15 style buffers that can be found in any local gun store, not the harder-to-find .308 shorty buffers. This makes it easier to fine-tune buffer weights, especially if you already have some spare AR-15 buffers in your parts bin.
On the recommendation of the Stag rep, I installed an A5-length adjustable buffer tube as well as a Spike’s T2 buffer and Sprinco Orange spring. The slightly heavier buffer (4.0oz compared to 3.8) and much stiffer spring helped to tame the harsh recoil and improve feeding reliability somewhat — if a rifle is right on the edge of functioning reliably, this might be enough. In my case, it only reduced misfeeds from every 5-10 rounds to every 20-30, and the tip deformation issue was still present.
Studying the Gas System
During the troubleshooting process, I realized something about this rifle. Although it has an 18-inch barrel — which would typically be matched with a rifle-length gas system — the gas system is actually mid-length! This is the case for all Stag 10 rifles, both 16-inch and 18-inch.
After removing the gas block, I checked the gas port size on Stag’s factory 18-inch Ballistic Advantage barrel, and it measured approximately 0.060 inches. That’s surprisingly small, and it seems to be an attempt to mitigate the overgassed nature of this configuration. Given the combo of a short gas system, relatively high-pressure 6.5 Creedmoor round, and a suppressor, it’s no surprise this rifle ended up overgassed.
Would an adjustable gas block fix this? Maybe, but I believe a mid-length 18-inch 6.5 Creedmoor rifle is inherently harsher and more prone to malfunctions than a rifle-length setup with the same barrel length. So, I decided to simply upgrade to a new barrel and gas system.
Barrel Upgrade
After doing some research, I selected an 18-inch, Nitride-finished 6.5 Creedmoor Hybrid profile barrel from Criterion. I’ve had great experiences with Criterion barrels on other ARs, including a 13.9-inch CORE setup that’s an absolute tack driver, so I knew this one would offer the precision I was looking for. It features a rifle-length gas system and gas port that’s “tuned for proper function both with and without a suppressor.” It’s also a DPMS-style design, which matches my Stag receiver’s compatibility. I measured the gas port and found it to be approximately 0.081 inches, which seems fairly typical for 6.5 Creedmoor barrels with rifle length gas systems.
Criterion also offers a JP Enterprises headspaced bolt option, which matches this barrel’s JP barrel extension. I selected this option, as well as the Badger Ordnance TDX non-adjustable gas block and a BCM rifle-length gas tube.
Removing the old barrel was simple enough. I used a Midwest Industries .308 Upper Receiver Rod and bench vise to securely support the upper and barrel, then loosened the barrel nut using an armorer’s wrench and long breaker bar. Applying a little heat with a propane torch is also helpful if the barrel nut won’t budge. Once the new barrel, bolt, and gas system were installed, I headed to the range to test my AR-10 again.
Rounds Downrange & Final Thoughts
I began testing the new setup with the same Norma 143gr 6.5 Creedmoor ammo I had been using previously. It immediately ran better. Not only were there no more failures to feed, the rifle’s recoil impulse felt noticeably softer, and brass ejection was gentler too. I also tested some Hornady American Gunner 140gr ammo, which functioned properly and grouped slightly better than the Norma.
However, the rifle was still ejecting forward more than I’d like, with brass landing at 1:00 from the muzzle direction. This is an indicator that even with the new barrel, it was still somewhat overgassed, likely due to the increased backpressure from the older 3D-printed Daniel Defense suppressor.
For the last bit of fine-tuning, I finally relented and installed a Superlative Arms adjustable gas block, which can be set to either restrictive mode (like a traditional AGB) or bleed-off mode (venting excess gas out a port on the front of the block).
I turned the adjustment screw a few clicks in the bleed-off direction until the brass was landing at roughly 3:30. In comparison to a traditional restrictive AGB, the bleed-off function produced less gas to the face and will reduce the amount of carbon fouling to keep the rifle cleaner. It also allowed me to swap the extra-stiff Sprinco Orange spring back to a factory .308 buffer spring for a little less felt recoil. While I didn’t initially want to install an AGB on this rifle, I’m glad to only use it as a finishing touch rather than a hasty fix for a much larger reliability issue.
Although the AR-10 (and its derivatives, like the DPMS LR-308) can be tricky to get running right, especially suppressed, a little research and troubleshooting will take you a long way. Don’t just slap an adjustable gas block on your rifle and assume it’ll solve all your problems. Sometimes that’s all it takes, but an adjustable block can’t make an undersized gas port any larger or completely negate all the effects of a short gas system. Do your homework, test modifications one at a time, and stay consistent with variables like ammo and magazines. The result will be a rifle you understand better and can rely on with confidence.
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