April 12, 2018
AR-15s can be obscenely accurate. The design lends itself to consistency, making it an anomaly in the semiautomatic rifle world.
Perhaps the primary reason is that unlike other semiauto rifles, the standard gas-impingement AR-15 doesn't have a bunch of moving piston parts hanging off its barrel - meaning it can effectively be free-floated.
However, a lot of AR-15 rifles and carbines aren't all that accurate - certainly not as accurate as their owners hoped. Several things can be done about that.
Recently, I built a very light 18-inch AR with a premium barrel by Proof Research. My intent was to create an AR-15 with DMR (Designated Marksman Rifle) capabilities coupled with all-day carryability -and for a desk-softened, middle-age fellow like me, that means something different than for a battle-hardened warrior.
I ordered a Battle Arms Development (BAD) BAD556-LW receiver set through my local gun shop and ordered the Proof Research barrel to be shipped to my home. Unfortunately, when you order parts, you can't try them for fit, and the barrel wasn't as tight in the receiver as I prefer. However, I forged on and assembled the rifle using simple tools in my garage - no special treatment whatsoever - and accomplished my goal of a lightweight AR (it weighed in at 5½ pounds) that produced 1-MOA accuracy with at least one match load.
To my dismay, although one load, Barnes' 85-grain Precision Match, shot a whisker under 1 MOA, the other ammunition I tested averaged closer to 1½ MOA. That's disappointing accuracy from a Proof Research barrel. However, past experience with Proof told me that the issue was most likely not the barrel. More probably, it was the loose fit between barrel and receiver.
So, I tore apart the entire build and reassembled it using every accuracy-enhancing trick I could dredge up.
Squaring the Receiver
In his article "Barrel Installation: do it once, do it right" in the 2016 Issue No. 3 of Guns & Ammo's Book of the AR-15, gunsmith and competitive shooter Patrick Sweeney writes, "If it is loose, you will have a casually accurate barrel at best."
Taking the concept a step further, LanTac USA's Brett Stewart told me, "To get the best possible accuracy, you should have to freeze the barrel overnight and heat the upper receiver with a blow dryer before fitting them together. Even then, the fit should be really, really snug."
Well, my barrel wasn't snug, and I didn't have the luxury of dry-fitting various upper receivers until I found a tight one. So I decided to employ an old National Match armorer's trick and Loctite the barrel in -which Proof Research suggested from the start. With fairly comprehensive data from the carbine as built, the project made for an interesting before-and-after test opportunity.
In addition to Loctiting the barrel into the upper receiver, Proof Research's marketing manager Chris Polley suggested three other accuracy enhancements:
1. Lap the face of the upper receiver to square it perfectly.
2. Tune the gas tube for minimal contact.
3. Use a supported bolt carrier group.
If I was going to square the receiver, it had to be done before Loctiting the barrel in place. I ordered an AR-15/M16 Upper Receiver Lapping Tool ($35) from Brownells. With it chucked in my power drill, I lubed the body of the lap and applied polishing compound to the face of the lapping surface per instructions. According to Brownells, 180-grit aluminum oxide compound is ideal, but I only had 600-grit. It took 60 seconds instead of the recommended 15 seconds of lapping but worked just fine.
Lapping the Battle Arms Development upper receiver provided peace of mind, but it probably didn't need it. The face of the receiver polished up perfectly concentric and square without any sign of unevenness. Brownells recommends lapping until 80 percent of the receiver face is bright and squared; 100 percent of the BAD receiver brightened evenly under the lap.
Time for Glue
Wondering what type of Loctite to use, I put in another call to Polley at Proof Research. They use Loctite 620, which is a special high-viscosity, industrial-grade Loctite designed for setting bearings and other cylindrical devices. Unlike typical red or blue Loctite, which is designed for optimum performance in tolerances of .005 inch or less, the green 620 is optimized for tolerances up to .015 inch (.381mm). Additionally, it has a much higher temperature rating, shrugging off temperatures up to 450 degrees Fahrenheit (232 degrees Celsius), and has an admirable sheer strength of over 3,800 pounds per square inch (psi).
Stewart, however, opined that Rocksett is a better barrel-bedding agent, indicating that the ceramic compound better resists extreme temperatures. Amazingly, Rocksett is rated for heat up to 2,015 F (1,100 C). It's also nontoxic, odorless, nonflammable, and resists oils and solvents - all nice benefits. However, its sheer strength (460 psi) is less than that of Loctite 620. Plus, it's a low-viscosity fluid rather than high viscosity like the Loctite, potentially making it more difficult to control.
Unsure which compound to use, I ordered both. After truing the face of the upper, I aggressively degreased it and the shank of the barrel with Hoppe's aerosol cleaner/degreaser. Then I carefully applied anti-seize grease to the receiver threads to prevent galling when torqueing the barrel nut and to act as a release agent if I got glue where it wasn't supposed to be.
Still undecided on whether to use Loctite or Rocksett (both of which would undoubtedly work just fine), I put a drop of each on my finger. The higher viscosity of the Loctite impressed me. It sat in a nice droplet on my finger while the Rocksett ran off like water. The stiffer fluid would make it easier to achieve a full fill in the gap between barrel and receiver. Plus, Loctite 620 is a rich green and would photograph more clearly.
Applying Loctite 620 liberally around the barrel extension, I slowly settled it into the receiver, rotating it back and forth a bit until the indexing pin dropped into its slot and the barrel was in place. Green Loctite oozed out of the joint between barrel and receiver, but a quick glance inside the receiver showed that none had crept inside.
Using my Brownells AR-15 barrel extension torque tool and the supplied wrench, I tightened the BCM barrel nut and stood the assembly muzzle upright to cure.
Tuning the Gas Tube
Vibrations caused by overly firm contact with the gas tube can negate the free-floating characteristics of your AR-15 barrel. Tuning the gas tube for minimal contact through the receiver and in the gas key atop the bolt carrier group (BCG) not only reduces its potentially adverse affect on accuracy, it can aid reliability and smooth operation.
Tuning a gas tube is a trial-and-error process. Dry-fit your gas block and tube assembly, noting which side of the channel through the upper receiver the tube bears against. Remove the gas block and tube, and carefully apply a little bend. The gas tube is pretty resilient and won't want to bend, so beware of getting frustrated and cranking on it too hard. That usually results in too much adjustment and a trip to the gunshop for a new tube.
If your gas block is secured using locking screws rather than a pin, and the barrel isn't drilled with divots to locate said screws, you'll need to be sure that the gas block is oriented straight each time you dry-fit it.
The process can take a fair amount of time, and you'll need patience. If you're cautious, eventually you'll have the receiver tube tuned to where it basically floats inside its channel into the receiver.
Next, disassemble the BCG and install the carrier without charging handle or bolt, thus reducing any potentially distracting contact inside the receiver. With the tip of a finger inside the rear of the carrier, ease it forward, watching and feeling the gas tube as the gas key (which you can't see) contacts it and slides over it. Ideally, although there will be a slight click as the gas key engages the gas tube, there won't be a discernable bump or resistance, indicating that the two line up well.
On the other hand, if you can feel a definite bump as the gas key contacts and slides over the gas tube, and can see the tube shift to one side or the other, you've got work to do.
Carrier bodies have fairly generous tolerances built in, and it's possible to rotate them a degree or so each direction from center. With the finger you've got planted up its rear (forgive the horrible metaphor), turn the carrier as far as it will go in one direction and repeat the gas tube/gas key fit test, then turn it fully the other direction and test it. If the gas tube still enters the gas key smoothly, you're good to go.
If not, you may have to get creative. In his aforementioned barrel installation article, Sweeney suggests that a screwdriver can be helpful in persuading the gas tube and gas key to line up and play nicely — stick it inside the upper receiver and use it to carefully apply a slight bend to the tube in the needed direction. Again, overenthusiasm can result in a trip to the gunshop for new parts, so proceed carefully and test often until you're satisfied that you've minimized gas tube/gas key contact as much as possible.
However, with your receiver face properly squared and the gas block installed perfectly straight, the need to apply inside bend to the business end of a gas tube is minimized. This is particularly true with mid- and rifle-length gas systems because the longer reach from gas block to receiver channel effectively minimizes the potential for a cockeyed gas tube.
With the gas tube for my lightweight DMR tuned to dry-fit perfection, I degreased the inside of the gas block and the barrel journal and carefully installed it with the same green Loctite 620 that I used to bed the barrel into the receiver. The Loctite was probably unnecessary, but I reasoned that bedding the gas block couldn't hurt.
Installing a Supported Bolt Carrier
Pop the rear receiver pin and open any AR-15 with a standard bolt carrier, put your thumb against the rear of the carrier, and wiggle it. You'll find a surprising amount of play. According to Polley, too much play reduces your AR-15's accuracy potential.
Stewart pointed out that Proof Research is really an aerospace company, and its engineers have testing and measuring instruments at their disposal far beyond those available to most firearm designers. According to Stewart, Proof's R&D department found and was battling measurable group shift between rounds coming from the left side of an AR-15 magazine versus those feeding from the right side. (We're talking ridiculously tiny overall groups that most companies would be more than delighted with.) But Proof Research reasoned that if measureable, predictable shift was occurring, there must be a way to eliminate it.
As you can see, a typical upper receiver measures right around an inch inside diameter - exactly .994 in the case of my Battle Arms Development upper. A standard bolt carrier measures .93 inch, leaving .065 inch of play. As the buffer spring slams the BCG forward on a fresh cartridge, the carrier cocks slightly right or left, depending on whether the round feeds from the right or left of the magazine, leaving the bolt ever so slightly off square. Lantac's Enhanced BCG has a slight step up at the rear of the carrier - termed a "boss" by Lantac - which increases the overall diameter to .978 inch and reduces play between receiver and carrier to about .015 inch. That's 75 percent less slop. The result? Straighter, more consistent bolt orientation and increased precision.
Now, you can argue that champion High Power competitors have been successfully using standard-diameter bolt carriers in their National Match rifles for decades, and you'd be correct. I've had AR-15s that were consistent half-MOA shooters with standard-diameter bolt carriers. But in our ongoing search for greater accuracy, any step that quantifiably reduces inconsistency is worth taking. Besides, carefully built AR-15s fit with Proof Research barrels often provide quarter-MOA accuracy, and believe me, no stone can be left unturned when working to achieve that level of precision.
The great beauty of installing a supported bolt carrier group by Lantac is that it's effortless and foolproof; simply remove your standard BCG and slide in the Lantac. (Don't forget to check headspace before firing.) The Enhanced BCG will leave your wallet $285 skinnier (military and LE discounts are available), but it is of superb quality: automatic rated, precision machined from 8620 steel (carrier) and Carpenter 158 (bolt), shot-peened, magnetic particle inspection (MPI) tested and nickel boron (NiB) coated to a surface hardness of 82 to 85 Rockwell C. The bore of the carrier and gas key are hard-chrome lined.
Range testing: Before and After
To prove or disprove the effectiveness of my accurizing efforts, I chose three of the same factory loads I tested through the rifle prior to the gunsmithing, plus a Black Hills 40-grain V-Max load that I intend to use on predators.
Although I've typically had superb results from Black Hills' 60-grain V-Max ammunition and like the load for personal protection purposes in urban areas, this rifle didn't initially shoot it particularly well, averaging 1.44 inches for three consecutive five-shot, 100-yard groups without allowing the barrel to cool. I chose it as the first load to test through the newly accurized AR-15, and it averaged .74 inch — for practical purposes, half the previous average.
Switching to Winchester's 69-grain SMK load, I proceeded to shoot a .34-inch five-shot group. Delighted, I fired two more groups. Together, the three groups averaged .54 inch - well under half of the 1.26-inch average the same ammo produced prior to accurizing.
To my surprise, the rifle's previous favorite 85-grain Barnes Precision Match load benefited little from the accurizing procedures, turning in a .95-inch average. Still a shade better than the previous 1.04 average, but not by much.
However, the 40-grain predator load averaged .59-inch groups. Wonderful performance that puts to lie the claim that a 1:8-inch twist barrel won't shoot light bullets well.
One characteristic of the 5 pound, 8 ounce AR-15 is worth mentioning: It's not easy to shoot with perfect consistency. There's a world of difference between it and my 14-pound National Match AR-15 or a heavy varmint-type AR-15 fit with a configurable Magpul stock. It's easy to carry but hard to shoot precisely. Really, in providing outstanding accuracy with no discernable point of impact shift over long shot strings, it's almost miraculous considering its light weight. Candidly, I believe it's capable of greater precision than I'm personally able to milk out of it because it's harder to hold stable than a heavier rifle.
The accurizing procedures basically halved average group size. Before, assembled using what I call "garage build" tools and techniques, no load averaged less than 1.03 inches. After, no load averaged over .95 inch, and two of the four loads tested produced five-shot groups between ½- to .6 inches, including hot-barrel groups.
With testing complete, I mounted my SilencerCo Harvester suppressor, which I like to use while predator hunting, and tweaked my zero to perfection with the 40-grain V-Max bullet. Surprisingly, point of impact shift was only about 1 MOA down, with no horizontal shift; previous to Loctiting the barrel in place, it averaged 2½ MOA down and 1 MOA right.
Are the accurizing procedures worth it? If you are unhappy with your AR-15's precision - or lack thereof - the answer is a resounding yes. Aside from the $285 Lantac Enhanced Bolt Carrier Group, the complete process will set you back less than $50 and an afternoon.
My only complaint with the process is this: I used to be able to assemble a complete AR-15 - every part and pin - in less than two hours. Now? It's more like five or six hours plus cure time for the barrel bedding. The results of the accurizing procedures were so compelling that my OCD streak won't let me build an AR without them.
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