AR-15 barrel harmonics: an approach that can

cut your group size by about 20%.

Physics Nobel Laureate Richard Feynman once said, "If you think you understand quantum mechanics,
you probably don't understand quantum mechanics." I believe most shooters, handloaders, and
gunsmiths could substitute "barrel harmonics" for "quantum mechanics" here for an accurate assessment.
Although I had to take physics classes in college, barrel harmonics weren't discussed. I won't arm wrestle
a physicist pointing out some of the points in this article aren't due to barrel harmonics, but I do know they
will significantly tighten groups.

The downside of these techniques is a time and range firing effort to establish correct settings, angles,
and pressures. These are not simple "drop in" solutions and many won't have the time for testing. Some
of these modifications require gunsmithing and this will help maximum accuracy tuning.

For decades, shooters have tailored handloads for optimal accuracy with different powders, charge
weights, bullets, and bullet seating depth. I perform a load work up for every rifle I build or rebarrel and
the customer receives a box of that ammo which will yield good results if properly duplicated. The ammo
approach doesn't improve harmonics much, it's tailoring ammo to perform with the gun as is. Not
everyone can tune ammo. Factory-only ammo and requirements of magazine-fed rifles allow less
variables.

American Rifleman published an article about varying the torque on the pillar screws of a pillar bedded
rifle to tune for better accuracy. Although I primarily work on Stoner platform rifles lacking conventional
stocks and bedding, I sought a similar approach. Torque can be varied on the barrel nut. Begin by
properly stabilizing the barrel extension in the upper receiver. Note, Stoner designed the gas tube to pass
through one of the barrel nut's "teeth" and too much or too little torque on the nut results in a bent gas
tube, causing the bolt carrier key to misalign. To avoid this, grind down the offending tooth on the barrel
nut and hit the area with a little touch up. This doesn't weaken the barrel nut and it can't unscrew because
the next raised tooth would be stopped by the gas tube. I have performed this minor modification to
hundreds of ARs without incident. Assembly is finished with an initial torque of 47 ft-lb for heavy barrel
(one inch under the handguards) National Match 20" Service Rifles and 26" Match Rifles. Rack grade,
skinny barrels use an initial torque of 60 ft-lb.

Any time the barrel nut is timed, run the nut forward to whatever position or torque desired or a bit
heavier, back off about a 1/4 turn, then to the desired torque or setting. Some gunsmiths advocate
checking for a "false stop" found with a slight machining burr or imperfection in either the barrel nut or
receiver thread that stops the nut prematurely. I've only encountered this problem once with a Team rifle
in Australia where the barrel nut appeared about one notch short. The upper receiver was a discounted
factory blemished piece that zeroed and shot well once this procedure was done.

Also, a removed, offending barrel nut tooth will be concealed by the delta ring assembly and handguard.
Bison Armory (bisonarmory.com) offers their AR-15 Barrel Nut Shim Set with three 0.001" shim washers
that fit on the barrel extension to influence the timing of the barrel nut.

Use of shims in AR gun plumbing has increased. Peel washers to time flash suppressors came first, then
shim washers were used to correct windage slop in rear sights. Ron Power designed shim washers to
reduce accuracy-robbing slop in the front pivot pin. Bison Armory's offering is the next step. I believe
there is room for more fixes via shim washers in helping deal with timing up barrel extensions to existing
gas ports in AR barrels.

Before leaving the shop, attach anything that will normally be on the barrel before testing, such as a large
target front sight. Given a six ounce weight, it could likely influence harmonics. Likewise, the back of the
barrel must also be considered, such as with bolt carrier weights. Start with ten round groups with known-
quality ammunition, preferably with what the shooter intends to use. Adjust the torque setting by 10 ft-lbs
after each group if needed. It doesn't matter if the initial adjustment is up or down. My range is next to my
shop, but a portable vise C-clamped to a shooting bench works otherwise. If the torque change yields a
smaller second group, you're in the correct direction. Otherwise, go the other way. Let the results dictate
the adjustment. Continue testing and re-torquing until the groups start to open up to find a sweet spot. I
don't go below 27 ft-lbs and add Blue 242 Loctite any time I go below 30. Having made a graph of these
settings from various rifles, the average setting is 47 ft-lbs for heavy barreled guns and 60 for skinny
barrels, though I've had a few that went as high as 77 and 90 ft-lbs, respectively.

Torque changes on issue guns is easy because the handguards can be removed to access the barrel nut.
Likewise, National Match guns with 11/4" Compass Lake float tubes have easy barrel nut access. Most 1
3/8" National Match Service Rifle float tubes are two piece and require time consuming partial
disassembly to get the wrench on the barrel nut. The front sight assembly has to be freed to allow it to
move forward, then the actual float tube body must be unscrewed so the wrench can be placed on the
barrel nut. Once the new torque has been applied, the float tube is reassembled and the front sight
located to its original position. Given this added hassle, I swap 1 3/8" float tubes temporarily for a 11/4"
tube for testing, replacing afterward. Worse than the 1 3/8" float tubes, many Match Rifle and tactical
handguards will not let you change or measure torque. I've found the JP Enterprises Modular Hand Guard
System to be a user friendly choice.

Notice I suggested torque changes in 10 ft-lb increments, which may seem too coarse for fine tuning.
Let's say we test a skinny barrel A2 upper with an initial setting at 60 ft-lbs, then try 70 for group two. If
the second group is better, 80 ft-lbs is next. Should that group get fatter, it's simple to conclude 70 ft-lbs is
the sweet spot. You're free to try further test groups in 5 ft-lb (or less) increments but I've found 10 ft-lb
changes to be all the guns can "see."

Regardless if you're building a Service Rifle, Match Rifle, or tactical rifle, expect to harvest about a 20%
reduction in group size by following these steps. I've lamented that Stoner's barrel nut design is flawed for
maximum accuracy. This method of barrel fastening is fast and easy but promotes accuracy-robbing
movement of the barrel extension inside the upper receiver. As long as these rifles attach the barrel with a
barrel nut it will be difficult to achieve maximum potential accuracy that the barrel makers are giving us.
Varying torque on the very thing that limits our accuracy allows some significant accuracy strides.

One last note here. I described how to slightly modify the nut by removing a tooth to allow a range of
torque settings. A2 handguards time up to the barrel nut. Depending on the amount of torque that the gun
wants, the top handguard vent holes may be positioned either a little clockwise or counter clockwise from
the normal 12 o'clock position. No big deal to the shooter because the handguards are round but it can be
a big deal to the gas tube because it passes through a narrow channel in the back of the top handguard.
Neither side of that channel should touch the gas tube because that can throw it out of alignment with the
bolt carrier key. If you find either side of that channel crowding the gas tube, relieve it with a file or
grinding wheel.

Many shooters lack the gunsmithing skills and tools to make torque changes on the range. Not to worry!
There are other techniques that are more user friendly and offer similar accuracy gains.

Old School Tech

Prior to Viet Nam, I spent my youth hunting deer. I still remember stories from the woods about some
hunter's deer rifle shooting terribly and some smart guy fixed it with a section of match book cover
strategically positioned in front of the action where the barrel attached or near the front of the stock.
According to lore, the subject firearm magically turned into a tack driver. As with many of these urban
legends, nobody ever stepped forward to proclaim personally witnessing this. In spite of whatever
enhanced accuracy those matchbook-cover gunsmiths achieved, the deer population easily survived.
Reading Harold R. Vaughn's Rifle Accuracy Facts reminded me of these matchbook modifications. Dr.
Vaughn discusses a modification he called "The O'Connor Bedding Approach" involving two epoxy-
coated cardboard shims positioned at 4:30 and 7:30 at the front of a wooden rifle stock. Dr. Vaughn
thought it a good idea from a physics standpoint but had concerns about environmental conditions
changing the pressures on the barrel, with such warping moving the rifle's zero. I strongly second that
motion and predict changes in the grouping characteristics, however, I confess this gives some credence
to that old hunting lore.

The shim idea ties in with varying torque on pillar-bedded stock screws. Aftermarket Innovations
(503/792-3251, Stocks-Rifle.com) is marketing and installing their SmartStock Accuracy System, a tuning
device that mounts in the barrel channel near the front of the stock and applies upward pressure against a
specific section of the bottom of the barrel. The position of that support is adjustable and the shooter fires
groups to find the tightest while moving the support block in 1/64" increments. Because all their testing
was done on a bench with sandbags, I wrote in concerning issues when using a tight shooting sling for
support as this will pull on the stock and could alter the effect of the support device. Unfortunately, they
never responded. It's doubtful this was taken into consideration.

My Anschutz squirrel rifles have double pillar bedded fiberglass stocks by a major custom stock and
floated barrels. On the range, I found a tight hasty sling opened a visible gap between the bottom of the
barrel and barrel channel in the stock, returning to normal when sling tension was lessened. Note, this
applies to a specific shooter using the amount of sling tension comfortable for him. Stocks are made of
different materials and different dimensions and different firearms may well react differently to sling
tension. Every shooter using a sling for support will have individual sling tension to provide the amount of
steadying affect needed. I haven't tested the SmartStock so don't misinterpret this as criticism, however,
these are issues that can change accuracy and zero.

Firearms have more similarities than differences. I've made modifications to AR-15s based on what I've
learned from bolt guns and pistols. The SmartStock has potential and I gave the idea thought for some
modifications on certain ARs. The first is float tubes on National Match Service Rifles. These have metal
tubes with a barrel nut and delta assembly on one end and a sling swivel on the other. Using a tight sling
on an as-issued setup causes a temporary bend in the barrel and the float tube removes that. National
Match tubes fit under modified handguards that appear stock and the bottom handguard has vent holes
about 5/8" center to center. With a bottom handguard in place, I traced the vent holes with a white
Sharpie onto the tube beneath it, then removed the handguard and centered drill holes on the traced
patterns. I drilled seven holes corresponding to the most rearward vents to locate the barrel supports far
back to minimize flex in the metal tube. I consider this a design improvement over the SmartStock with
support near the front of the stock where flex will be greatest if a sling is used. I tapped the holes at 8-32.
This is the same thread normally used on wind-age-adjustable front sight housings and customers will
already have the appropriate 5/64" Allen wrench. 8-32 Allen screws purchased or dressed to about 9/64"
in length fit flush in a 1 1/4" float tube. A fatter 1 3/8" float tube needs set screws to measure about 13/64"
to 1/4" long. Longer set screws is OK unless the shooter uses a lead weight in the handguard, requiring
clearance holes in the lead.

For testing, the shooter locates the support screw in the rearmost hole and torques it tightly against the
bottom of the barrel. It is important to use the exact same torque each time the under-barrel support
screw is moved. I recommend 15 inch-pounds. Fire a group, then move the screw forward to the next
position and retest for all seven support holes. This initial range session makes bold 5/8" movements of
the support set screws. Those support holes are centered on the bottom handguard vent holes and the
handguards don't have to be removed to change the position of the support screws. As before, continue
to test as groups improve and then go past the sweet spot. Fine tune as desired. Let's say groups shrink
from the location furthest to the rear to hole 4 and then begin to get progressively bigger again with holes
5 through 7. Secondary holes drilled between 4 and 3 at the rear and another between 4 and 5 at the
front will help fine tune the performance. By firing additional test targets at these two secondary positions
the shooter should be able to nail a sweet spot. From my experience, trying to fine tune beyond these
secondary holes is a waste of time but you are free to experiment on your own. Getting set screws into
the secondary holes will require removal of the bottom handguard.

Another good handguard is the JP Enterprises Modular Hand Guard System. I use these on Match Rifles,
F-Class rifles, and tactical shooters can attach rails to all four sides. This unit allows ready access to the
barrel nut for tuning. Each handguard has two long 3 3/4" slots milled in the bottom for adjustable hand
stops via included dovetail inserts drilled and tapped to accept quick-detachable sling swivel studs. For
accuracy tuning, I do away with the sling swivel stud and substitute a 10-32 set screw with about 1/2" long
to fit the thread of the original dovetail and move it forward and back in the rear slot in the hand guard to
accomplish the tuning process. My substitute screw is long enough to reach and snug up on the bottom of
the barrel, supplying the tuning support. I take a piece of masking tape and attach it next to the slot and
mark off the same 5/8" tuning intervals as described above, yielding five tuning increments. Testing is as
previously described. Once a coarse sweet spot is found, move the dovetail any distance forward or back
from that setting to further fine tune performance.

While the JP is user friendly for tuning, other hand guards may be less compatible to this tuning method
but I'm sure you will apply your own ingenuity to make it work. Some handguard designs maybe
hopeless. In that event, employ other accuracy tuning methods.

Before closing my research on the location of the barrel support, I had to perform one final test. Both the
SmartStock and O'Connor bedding approaches located their respective barrel supports under the front
where they would be most susceptible to sling tension. Most of my customers are National Match
shooters using tight loop slings to steady their rifles, so I locate barrel supports near the back of the
barrel. Even though the float tubes and metal handguards in my guns are more rigid than wooden rifle
stocks, I wanted supports to the rear where any flex caused by heavy sling tension would be minimal.

In the interest of being thorough, I tried an experiment with the support located in the front half of a float
tube and shot test groups at 15 inch-pounds of torque. I was able to locate a sweet spot in the front half of
the float tube and I could get the groups as tight as the ones with rearward support, but no tighter. Using
the existing holes in the lower handguard, I tapped a total of six screw holes at 5/8" increments as before
and tested all six locations. What was noteworthy was the impact on the downrange location of the six
groups. The group shot with the under barrel support closest to the front versus the most rearward
location six holes back moved the point of impact by 7.5" at 100 yards! As one would suspect, the zero
shift which occurs by supporting the barrel at its front is much greater than providing support at the rear of
the barrel. Given the groups were no better with front barrel support compared to rear support, the better
choice is obvious.

Varying Barrel Support Torque

Some bolt gunners vary torque on pillar bedded stock screws in bolt rifles to tighten groups. Varying
barrel nut torque on AR-15s also tunes performance. Next step is varying barrel support torque. You will
need a quality torque wrench calibrated in inch-pounds. I like to use a wrench measuring down to 10 inch-
pounds or less. Many inch-pound torque wrenches are set up for 1/4" drive. If not, purchase appropriate
conversions, then use a Brownells Magna-Tip on the end to match whatever set screw or screw head you
choose for barrel support. I use 15 inch-pounds of torque when I am moving forward or aft with my
support to establish the correct location for it so that groups already fired don't need to be repeated.

Once you have located a sweet spot along the bottom of the handguard, perform the torque tests with the
support in that sweet spot location. I like to shoot groups with torque increased and decreased in five
inch-pound increments (10 and 20) from the initial 15 inch-pound setting. Continue in the direction of the
better group until it goes past the sweet spot and gets fatter. Feel free to fine tune in smaller than 5 inch-
pound increments. I seldom go above 25 in-lbs to find the torque sweet spot. Usually, the torque sweet
spot is with rather modest upward pressure. Remember, don't try bending the barrel and don't get
hoggish with the torque.

Besides accuracy penalties, there are other disadvantages to applying too much torque to the barrel
support screw. Too much upward pressure on the support screw runs the danger of stripping the threads
out of the metal handguard as some of these are only about 1/10" thick. There likely won't be more than
three threads tapped depending barrel support size. On top of that, the metal used is soft and strips out
easily, though it's suitable for removing sling pressure from the barrel.

Even modest torques can make substantial point of impact changes. For example, I located a sweet spot
on a customer's Service Rifle about three inches forward of the delta ring. Varying torque from 15 to 25
in-lbs moved the POI by four inches at 100 yards! It's important to verify zeroes after any tuning effort.

With the sweet spot torque identified, coat the set screws with red Loctite and torque against the bottom
of the barrel. If the set screws vibrate loose the sweet spot (and possibly zero) is lost. Flush-fitting set
screws can be staked like the bolt carrier keys screws, however, this will make removal more difficult
when rebarreling is needed.

The Other End

Long before varying the torque on the barrel nut was understood, gunsmiths were experimenting with
flash suppressors to accomplish the same thing. Early attempts involved the A1 flash suppressor because
it is not directional. The back section of the suppressor was sawed off and used as a jam nut. Most of us
would make a white paint dot on the forward section of the suppressor for tracking purposes. Moving that
front section about one clock face hour at a time sometimes produced noticeable accuracy improvement.
Beginning at 12 o'clock, the sweet spot would be found before a complete revolution was made
regardless of the direction turned. Once the sweet sport was identified, the front section was held in place
while the back section was jammed tightly against it to lock the setting in. While this technique is faster
than varying torque on the barrel nut, the torque method results in greater gains. Many gave up on the
jam nut method and started using peel washers as each washer moves the suppressor about one hour on
the clock face.

An often used, non-directional suppressor is the Vortex from Smith Enterprises for .22 calibers with
standard pre-ban 1/2 x 28 threads as well as 6.8mm and .308 versions with different threads. These work
on any threaded barrel, such as my .22 Magnum Anschutz. As with Al suppressors, Vortex units hit a
sweet spot within one revolution and thin sections of standard peel washers make one hour changes. For
competition rifles, make sure visible modifications comply with the rules. Vortex units are not legal on
NRA/CMP Service Rifles.

With 20" pre-ban Service Rifles, I combine suppressor and barrel nut tuning. I initially set the solid part of
the A2 suppressor at 6 o'clock as originally designed, then tune the barrel nut. With that complete, I
remove one peel washer and retest with the suppressor at 5 o'clock and again at 7. This differing
suppressor setting is barely noticeable. If I nailed the barrel nut torque sweet spot, there is no
improvement at either 5 or 7 o'clock. I have gotten good enough at fine tuning barrel nut torque settings
that most rifles leave my shop with flash suppressors at 6 o'clock. I call this suppressor tuning the "Poor
Man's BOSS" and is based on Browning's Ballistic Optimizing Shooting System. Both work by effectively
lengthening or shortening the barrel.

This is different from a taper bore. For decades, barrel makers felt the best barrels had a subtle taper to
the bore and were a bit fatter in front of the chamber with a uniform taper at the muzzle. In practice,
achieving this is easier said than done. Makers of quality AR barrels believe flash suppressors put on with
too much torque crush the bore at the muzzle, a constriction thought to deform bullets as they pass
through.
I tested this theory when I first started working on ARs by pretesting factory barrels with suppressors
mounted with a hard torque, then removing and reassembling with modest torque. There was usually an
improvement in accuracy! It appears there is some "elasticity" in stainless and chrome-moly steel. Once
the crushing torque is removed, the metal rebounds somewhat, with non-chrome lined barrels responding
best. I posit that chrome is not as elastic and likely holds more of the constriction.

I wasn't the only AR researcher to discover this. Others tried to use this "Poor Man's Taper Bore" by
carefully controlling the amount of flash suppressor torque applied and slightly constrict the muzzle
without being excessive. In reality, the whole length of the bore was not tapered. The hope was to slightly
constrict the muzzle and harvest some additional accuracy but I haven't found this to work as well.
Perhaps I am not in the right torque range. Regardless, I haven't heard of others having much success
with this either.

After discovering accuracy loss from flash suppressors mounted with too much torque, it was common for
AR gun plumbers to overreact and put suppressors on just hand with lots of red Loctite. I guarantee this
will let the suppressor eventually work loose due to heat. A loose suppressor will remove any tuning
efforts and likely change the point of impact. After a couple of embarrassing incidents, I compromised with
three foot-pounds of torque combined with Loctite and haven't had a single suppressor come loose.

Some gunsmiths and barrel makers apply hand tightening with red Loctite to the other end of the barrel.
Big mistake! The barrel extension is heated by the hot chamber and applications quickly loosen. I had it
personally happen and the rifle tracked horizontally across the target as the barrel unscrewed from the
extension! The correct torque on the barrel extension is 150 ft-lbs and I encourage that whenever
possible. If a situation prevents proper timing and requires dropping torque, use plenty of red Loctite.
Never go below 75 ft-lbs.

BOSS Look-A-Likes

There are accuracy tuner devices based on the BOSS. Units such as the Harrell's Tuner Brake
(HarrellsPrec. com, 540/380-2683] incorporate ports to reduce felt recoil. Others, like the Hoehn Tuner
(HoehnSales. com, 636/745-8144) just provide accuracy tuning. The Hoehn clamps on and fits any barrel
with a 0.750" outside diameter, exactly that of a Service Rifle. The Hoehn can clamp on to pre-or post-
ban barrels and Match Rifle barrels stepped down to 0.750" at the front. The Harrell's Tuner Brake
threads on but initially didn't offer a unit fitting standard 1/2 x 28 threads. I approached them about this
and they made a unit to this size for .22 caliber. For larger calibers, consult the company. The Harrell's
unit is more streamlined than the Hoehn. The only design change I would make in the Harrell's would be
no drilled vent holes at the bottom between 5-7 o'clock. As is, there's a significant dust signature when
firing from prone. The Harrell's costs less. Given that it tunes accuracy and reduces recoil, it's probably a
bargain.

The Hoehn comes without instructions. My experience with tuning ARs helped reduce the learning curve.
Since this was not a permanent rifle, I didn't spend much time fine tuning. A few hours on the machine
rest convinced me there's much potential. The tuner cut groups by 30% and a little additional experience
could have likely yielded more. Mr. Hoehn says this unit is popular with .22 rimfire shooters. I was
sufficiently impressed with my initial introduction that I plan to include one with each dedicated .22 LR
Service Rifle trainer I build from now on. This is more user friendly than the barrel nut tuning method and I
think combining both could result in stellar performance. The unit is a bit bulbous, isn't legal for some
competition guns, and not for everyone. I have heard F-Class shooters are using these tuners. Although I
had no problems with it moving or coming loose, for a permanent mount, use green Loctite on the bearing
surfaces and blue Loctite on the clamp threads.

I tested the Harrell's device in the same manner, using ten round machine-rested groups with known
ammo lots and a standard A2 flash suppressor in the 6 o'clock normal position, and then substituting the
Harrell's. I have experimented with various screw-on recoil reducers over the years. Many of them are
quite effective in lowering muzzle rise but they're usually loud. The Harrell's was no exception, however,
unlike other recoil reducers that often reduce accuracy, this unit has adjustments to minimize this
problem. Average group size with the Harrells unit ran about 23% larger than targets fired with the
standard A2 flash suppressor. If you do everything else right and start with an accurate, premium barrel
one MOA groups are possible with good ammo. Some kinds of shooting requires a balance of accuracy
and speed. For customers needing improved recoil recovery and speed while maintaining one MOA
group potential, the Harrell's unit might be a good investment. For maximum precision, a normal
suppressor or flash hider might be a better choice.

Tungsten

Before leaving the front end of the barrel, it's a good time to cover an experiment I tried that didn't work.
For years, hoplophobes and other fools have viewed flash suppressors as evil. Shooters in areas stricken
with laws influenced by this silliness need something to cover up the threads of muzzles stripped of their
suppressors. I made simple thread protectors in stainless or black for this. While making them, I thought
of M9/Beretta 92 accurizing. These pistols normally lack a bushing and the slide doesn't support the
barrel's front. Gunsmiths initially cured this by threading the front of the barrel and screwing on a bushing
fatter than the barrel to lock up with the slide. Early designs used regular steel as the bushing material
before someone tried heavier and stiffer tungsten instead. While difficult to machine, tungsten is available
in several grades and only the "softest" grade works in the pistol applications. Pistolsmiths using the
tungsten bushings claim enhanced accuracy by improved harmonics. There is no doubt that stiffening the
front inch of the barrel in addition to hanging more weight out there does influence barrel harmonics.

As Armorer for the Army Reserve Shooting Team I outsourced some work to various specialty shops.
Most of my earlier armorer and gunsmith education was in revolver work for local police. As cops
migrated to semi-auto pistols I worked on 1911s and Beretta 92s. I had so much AR-15 work with the
Reserve Team I outsourced M9 accurizing to Dr. Nick at Mountain Competition (Mountain-
CompetitionPistols.com, 570/6881020.) The tungsten M9 barrel bushings were an option and some
USAR Service Pistol Team liked his guns, so I contacted Dr. Nick about tungsten thread protectors.
Pistols use a different thread than the AR-15 but it was close enough to allow a judicious "re-tap" to fit.
Mike Harpster (Dead-CenterSports.com, 814/571-4655), a local gunsmith here in Pennsylvania that I
outsource some of my work to, performed the machine shop modification. The tungsten thread protector
was notably heavier than the regular steel.

I pretest all the AR barrels I sell on a machine rest for accuracy. The ones that shoot tighter groups retail
higher than those shooting average. I make these test groups available to my customers so they can
select a barrel that is the best compromise of price and accuracy to meet their needs. I had just received
a shipment of Bartlein gain twist barrels and was finishing up their ten shot break in when the thread
protector project was coming to fruition. I ran three tests with known lots of ammo in the pre-ban
configuration barrels using a normal A2 flash suppressor, a regular steel thread protector, and the special
tungsten unit. A third of the barrels liked the A2 flash suppressor, a third shot best with regular steel
thread protectors, and the final third favored the heavy stiff tungsten units. My study of the tungsten
thread protectors found firearms to be individuals with some liking the heavy protectors and others not.
Since only a third of the barrels favored the heavy protectors I don't offer them for sale. Outside of a full
machine rest test, a customer has no idea if his barrel would be one of those favoring the heavier
protector and I won't sell an expensive piece of kit that my own study pointed to having two chances in
three of making the firearm perform worse.

Why didn't the tungsten thread protectors have the desired outcome of improved accuracy? An M9 barrel
is five inches long and the tungsten bushing occupies the front inch, or 20% of the total length. Contrast
that to a 20" AR-15 barrel and a 1/2-5/8" long thread protector covering about 2.5% of the total length.
Perhaps part of the answer is the longer barrel is too long for the tungsten to have a positive dampening
effect on barrel harmonics.

Other Considerations
The DPMS Lo-Pro receiver has thicker walls than standard uppers. That increased thickness runs all the
way up to the back of the barrel nut threads at the front of the receiver. The threaded portion at the front
of any AR receiver is very skinny and this area provides extremely poor support to the third of the barrel
extension that lives inside this area. The DPMS Lo-Pro and Sun Devil (SunDevilMfg.com, 480/833-9876)
receivers are fatter just behind the barrel nut threads where the other two-thirds of the barrel extension
resides. It is posited by some that the added thickness of these receivers provides better support for the
barrel extension and helps tame barrel whip--or at least make it more consistent--adding to accuracy.
Folks are using the AR-15 with heavy 90 grain bullets out to 1,000 yards and barrels have been getting
longer to harvest as much velocity as possible. Some long range shooters feel that barrels between 26"
and 32" are so heavy that the skinny receivers just don't provide enough support.

I have been experimenting with both company's heavy wall receivers and have seen accuracy
improvements even in the wimpy .22 LR rifles with standard 20" barrels as well as centerfire rifles. I want
to look at a little larger sample size before stating that as fact, however. The DPMS unit lacks a forward
assist or dust cover and Sun Devil offers their receivers with or without either or both. The DPMS is made
to mount on a conventional dimension lower receiver and looks fine. Sun Devil offers lower receivers that
are a little fatter than normal and their receiver looks best on their lower. That said, Sun Devil uppers are
compatible with normal lowers. Consult the rules of competition before using anything that isn't stock.
Heavy upper receivers would not be legal for NRA/CMP Service Rifle matches.

Conclusions

In the decade I served as the Army Reserve Shooting Team Armorer I was successful in cutting the
average group sizes of the Team's Service Rifles fully in half. When I left, every one of them shot sub-
MOA with good ammo. Some improvements required new parts, such as using premium match barrels
and substituting match bolt carriers. Budgets are always fixed, so in addition to buying new parts, I also
had to find less expensive alternatives to improving accuracy. Most of the tips in this article fall into that
category. They are cheap on the wallet but labor intensive and work on all Stoner platform firearms. Your
application of these tips will depend on how much accuracy your customers demand, your individual
skills, and how much time you have. No gun has left my shop without application of a combination of
these tips. I am sure they will significantly enhance the accuracy of the rifles you build as well.

I began this article with a quote by a Nobel laureate. Even though I've never been nominated for a Nobel
Prize, I'll end with a quote by me. "Shooters, handloaders, and gunsmiths tend to blame almost
everything they don't understand or can't explain on barrel harmonics."