Improving rack-grade AR-15s: achieving high performance from low-bid uppers.
A Strong Foundation
The first article I wrote for American Gunsmith was "The Relationship Of Accuracy To Barrel Extension Diameter In The AR15" in March and April 2013. I wanted that published first because barrels are held in the upper receiver of Stoner platform rifles by a barrel nut, which does an adequate job of reducing front to back movement of the barrel in the receiver but not as good removing sideways or up and down movement. In fact, there is so much movement you might be able to detect it with the naked eye. I first noticed this flaw while competing in Australia with the Army Reserve Marksmanship Program. In fairness, Stoner's design was never intended to be a match gun. We can fix this problem by fitting an extension to the receiver, ensuring we have a strong foundation on which to make additional improvements. I suggest studying that article for particulars but I'll review high points here. This can shrink average group sizes by about 35%!
Start by disassembling the upper down far enough to get the barrel off. Degrease both the barrel extension and the inside of the upper receiver. After allowing sufficient drying time (I recommend overnight) cut a piece of 0.001" thick shim stock about 3/4" wide and 2 1/2" long. Wrap that around the back of the extension insuring the gap between the ends of the shim stock occur in the up position, evenly spaced on either side of the extension pin. Try to slide everything back together. If things go together too easy you'll need to cut a slightly longer piece of shim. On the other hand, if you can't get the barrel extension back into the upper, take a little off of the shim ends. Continue by trial and error until you get a nice tight fit, then pull everything apart. This pre-fitting also develops "memory" in the shim stock that will make final assembly much easier.
Coat the entire extension and inside of the receiver where the barrel extension contacts with Loctite Green 620. Wrap the fitted shim stock back around the rear of the extension and coat with more green Loctite. Carefully slide everything back in place paying attention to movement of the shim stock. It may want to slide forward and if it goes too far it will curl up between the extension flange and the front of the receiver, resulting in a high spot that the barrel can pivot around. Pull everything apart and start over if you suspect this of happening. Loctite 620 is slippery and slow drying which facilitates getting things right. After everything is assembled wipe excess Loctite off the male receiver threads and install a barrel nut at a temporary torque of 60 foot-pounds. Clean up any excess Loctite on the inside of the front of the receiver and the back of the barrel extension. Allow sufficient drying time before firing. I prefer at least three days. After drying, time up the barrel nut so the gas tube and bolt carrier key are in prefect alignment. Getting this right in any AR-15 can substantially improved accuracy. I go into detail in "Gas Tube Alignment" (American Gunsmith, March 2014.)
While waiting on the Loctite to set, move to the other end of the barrel and correct things around the flash suppressor. Most flash suppressors are put on by gorillas. Whether the gun you are working on has the older peel washers or the newer crush washers (the term itself scares me) they're usually put on with too much torque. I suggest replacing a crush washer with peel washers and like having the flash suppressor back as far as I can get it to time up. Smart guys claim too much torque on the flash suppressor can crush and deform the muzzle and crown, causing bullet deformity. Some gun builders over react and insisted on only hand tightening and using Loctite Red 271 but I've found this works loose. I use a torque wrench at three foot-pounds along with the Loctite. That doesn't sound like much, but it's more than I can back out with a gloved hand and I haven't had one work loose yet. Needless to say, both the male threads on the front of the barrel and the female ones inside the flash suppressor need to be thoroughly degreased and dried before assembly the day prior in conjunction with the barrel extension and front of the receiver.
I have to admit that when I was first searching for the right amount of torque to apply to the flash suppressor my first attempts were higher, though not as high as the military-recommended 15-20 foot-pounds. I've had occasion to return a few NMC barrels to their manufacturers for follow up lapping. In some cases those barrels were assembled onto upper receivers and had flash suppressors attached at the slightly higher torques I was experimenting with. Lapping is a polishing process done by hand. It obviously adds time and cost to the end product so not all barrel makers do it. Those custom barrel makers that do have a real feel for what is right and what isn't. In every instance, regardless of the fact that different barrel makers were involved, I always received unsolicited comments noting a constriction at the muzzle. That's when I backed off my torque to the three foot-pounds I use today. I have machine rest tested with the heavy factory torque and then a lighter torque, revealing immediate improvement in group size.
Having bad mouthed excess constriction at the muzzle and highly-torqued flash suppressors, you may believe this is always bad. Not so fast! There was a theory tracing back to 19th Century barrel making that a very slight taper in the bore was a good thing, though I've never seen actual numbers for the beneficial amount. More recently, BlackStar, the folks in Texas that used to electropolish rifle barrels (a process that removed slight amounts of metal) with the goal of ending up with a uniform and very subtle taper to the bore slightly tighter at the muzzle. While the company folded due to an acid in their polishing process becoming so regulated that storage and disposition became too costly, the Army Reserve Service Rifle Team had used match barrels processed by them with success.
Both the Army Reserve and the National Guard shooting teams tinkered with this theory. I call it the "Poor Man's Taper Bore." It isn't a true taper because only the muzzle is manipulated. Abetter description might be "Controlled Muzzle Constriction." It is pretty intuitive that if five foot-pounds of torque on a flash suppressor constricts the muzzle a little then fifteen foot-pounds constricts it more. This led us to wonder if there was some optimal and repeatable amount of torque that would apply just the right amount of constriction to somewhat mimic the alleged accuracy enhancement of a true tapered bore. I fooled with research on the idea for a short spell and when I didn't see much coming of it I dropped it.
The National Guard team was a bit more thorough. That said, Derrick Martin, one of the Guard's smart guys and author of The Complete Guide to AR-15 Accuracy, mentions many successful techniques developed during this time period but nothing about manipulating flash suppressors. One reason for dropping the idea is likely because this can be rifle specific. One gun might want one foot-pound of torque while the next one prefers four, requiring time consuming testing and tuning of each specific rifle in a machine rest. Most of us would expect rifles with standard 0.219" lands and 0.2240" grooves to have different torque requirements than barrels with tall 0.218" lands and slightly deeper 0.2245" grooves. As far as I am concerned, the jury is still out on "Controlled Muzzle Constriction" but it might be an interesting project for a gunsmith with time and a quality rifle machine rest to pursue.
Note, the previous discussion was about controlling muzzle constriction using torque. That is not the same as controlling the position of the flash suppressor which is an entirely different science and one that I have nailed far better! All of us can picture the difference in a flash suppressor which is screwed onto a barrel with just a single 0.002" section of peel washer compared to the same suppressor and barrel with a 0.145" crush washer. In the first example the suppressor sits back further. This front to rear positioning of the suppressor influences harmonics.
Front Sight Taper Pins
A short movement rear on the barrel finds the next accuracy robber: Taper pins holding most issue front sight assemblies in place. Once again, these are hammered in by gorillas at the factory, degrading accuracy a bunch. Barrels rifled with the button method or hammer forging don't remove metal but displace it, leaving a lot of stress in the metal that has to be removed. Most issue A2 barrels are button rifled and stress relieved before the front sight assemblies are installed. According to theory, anything done to the outside of the barrel influences the inside of it. An example is fluting. If a button rifled or hammer forged barrel is fluted after being stress relieved this can result in dimensional changes inside the bore of the barrel, impacting accuracy.
Another example of changes outside the barrel resulting in internal changes involves the taper pins holding the front sight assemblies in place. These are driven in hard to insure they don't back out during rugged use. The theory says that doing so causes a constriction inside the bottom of the bore in line with each pin, so all bullets passing the first pin are deformed and then deformed a second time (about 1/7 of a turn) again as they pass the forward pin. Thus, the bullet is boogered twice and on different points on its circumference. This can degrade accuracy a fair amount. I have performed machine rest tests and found a noticeable improvement in accuracy by correcting this issue. Before discussing a fix, I can say you'll see the most improvement in stainless or chrome-moly barrels that aren't chrome lined as these seem to "bounce back" well. Chrome-lined barrels will also improve and need all the help we can give them but improvement may not be quite as noticeable as the chrome may not be as "elastic" as regular barrel steel so the deformities inside the bore don't iron out as readily.
There are two ways to do this. I will cover the most intuitive first. Drive out either one of the two pins. The second pin is left in place to keep the alignment of the front sight near top dead center where the manufacturer (hopefully) located it initially. Remember, these pins are tapered and only come out if hammered on from the left (bolt catch) side of the barrel where the pin is smaller. Support the barrel when doing this, such as using a Brownells AR-15 Front Sight Bench Block. This block is made of polyethylene and has some "springiness." For more rigid support turn a standard metal bench block upside down.
Once that first pin is out put some cutting oil on your taper pin reamer (Brownells #080-000-674AK or Size #0 from MSC Industrial Supply.) The reamer needs to be inserted from the fat side of the hole on the right (ejection port) side. Check your progress frequently to avoid hogging too much. Spray out the metal cuttings and make trial fittings by pushing the pin in by hand. You want to stop at a point where some modest raps with a small hammer are needed to secure the pin. Leave that fitted pin in place temporarily and drive out the second pin. After cleaning off the shavings from the reamer, apply more drill oil and use as before. When you are satisfied, completely disassemble and thoroughly degrease everything. After thorough drying time, use the green Loctite of your choice. Green 638 dries faster, while 620 is thicker in consistency and is the one I use if I get a little heavy handed with the reamer because it is designed to fill spaces.
Apply your Green Loctite to the barrel where the front sight bands will be, inside the bands themselves, on the pins, and squirt it into the pin holes. It is good practice to insert a spare gas tube through the triangular handguard end cap and into the front sight housing during the drying period, but be careful not to get any Loctite on it. It is common for some to migrate during assembly and it will often end up "welding" the end cap in place. Temporarily inserting the gas tube ensures that the hole in the end cap is properly lined up with the corresponding gas tube hole in the front sight housing. Remove excess Loctite from the barrel, front sight housing, and pins and allow a couple of days for everything to set up.
The second method is no more work than the first and is the technique I always employ on any "Go To War" gun I am working on because I believe it gives a firmer pin hold. Drive out both pins and remove the front sight housing. The Parkerized finish was applied to the barrel after the front sight was put in place. Even after the pins have been taken out you will need to beat the front sight off the barrel with some sort of non-marking device such as a wooden mallet. With the front sight housing removed, put a mild chamfer on the gas port. The chamfer will funnel gas into the corresponding hole in the sight housing in the event that the housing isn't exactly top dead center. If this is a previously-fired barrel that functioned flawlessly in the past you can skip this chamfering step.
Do not cut out any metal on the front sight housing at all with your taper pin reamer. Just insert the reamer in the taper pin hole in the barrel. Cut gradually and use frequent trial fittings of the pin to get the fit you want. This will give you full pin contact with the housing (on both ends of the pin) but will still relieve the bulge inside the bore that was caused by pins being hard driven into the holes in the barrel. Finish the process with a thorough degreasing of all parts, proper drying, and assembly with Loctite as described above.
So far we've concentrated on making improvements on stock guns without replacing any parts. Now we'll look at using upgraded parts.
Match Bolt Carriers
True match bolt carriers are usually fatter than stock carriers and have more bearing surface that is better polished than stock carriers, resulting in more consistent lock ups and potentially improving accuracy. Since the bolt carrier is locked up with the rear of the barrel when in battery it will influence harmonics and can improve or worsen group size. There is no way to predict this, requiring testing with the stock carrier and comparing to the match carrier.
Match carriers sold by Young and Les Baer have shiny, chrome-type outer finishes. Those by JP Enterprises have a black "self lubricating" finish (these require lubrication, however) that resemble an as-issued appearance. Regardless of carrier used, keep it clean, lubed, and it will function to its potential. Don't be afraid to add lube to the carrier during range sessions. When new, don't assume a replacement carrier's key will line up properly with the gas tube. Perform the diagnostics and alignment tasks to the gas tube as outlined in my 2014 article, "Gas Tube Alignment In The AR15."
If you really want to step up performance, replace the barrel. There are a number of barrel characteristics to consider when making a selection. A lot of research on this was done by competing US military teams back in the 1980s as militaries around the globe were adopting other types of competition shooting courses in addition to traditional bullseye to better train soldiers to survive in modern combat. Innovative new courses of fire were developed in Commonwealth countries, mostly in Britain and Australia. Time limits for the new matches were short and there was a lot of physical activity like running and obstacle negotiation. Many of the courses of fire got shooters off of their bellies and into other positions common to real combat situations. The National Guard and Army Reserve sent teams to both countries as well as Canada and South Africa to compete and learn.
In addition to being an author, Derrick Martin was one of the Guard's better shooters and is an accomplished gunsmith and owner of Accuracy Speaks (accuracyspeaks.com). Being competitors at the time, it wasn't until I read Derrick's book that I realized how parallel our research and conclusions were. Smart guys like Frank White, Albert Turner, and Bill Wylde had already made great strides with the black rifle by mostly working on bullseye guns and I freely admit to copying and adapting some of their ideas to rack-grade guns.
Here is what Derrick and I found regarding combat rifle barrels. First, twist rate. Issue barrels are currently rifled one twist every 7 inches. That is overly fast for most bullets. M855 stabilizes in 1:10 or 1:9 twist barrels. The faster 1:7 was adopted in an attempt to get better performance out of the tracer round which is longer than 62 grain ball and requires a faster twist. Another common bullet is M193 55 grain FMJ that the original M16A1 used. Stoner initially used a 1:14 in the first M16, which was increased to 1:12 because of concerns the slower twist might not stabilize bullets in extreme cold. At the time there was fear that Russia might invade us through cold climates in Alaska or Canada. I have shot 55 grain FMJ bullets in 1:12, 1:13, and 1:14 twists with stellar results in Pennsylvania winters. The lesson here is use the proper twist rate to stabilize the bullet you plan to fire. Don't over spin bullets, especially military FM] types. Doing so only exacerbates any ammo imperfections and makes guns shoot less than optimal. This is good advice regarding match guns and bullets as well.
I covered chambers and reamers in detail ("AR-15 Chamber Selection", August-October 2014) where I discussed the amount of free bore in NATO chambers. All the slop is warranted in dirty battlefield conditions but it certainly ruins accuracy. Both Derrick and I found that shortening the free bore resulted in better accuracy. The NATO chamber causes 62 grainers to jump at least 0.158" and 55 grainers to jump about 0.085". Both are like football fields in length from the bullet's perspective and any bullet not perfectly seated concentric with the body of the casing is going to enter the throat of the barrel at an angle when jumped those kinds of distances. A Wylde chamber shortens jump lengths to roughly 0.056" with 62s and 0.050" with 55s, increasing accuracy a bunch. I have never experienced any pressure issues firing issue ammo in Wylde chambers. According to Derrick's book, the Guard ordered a special reamer that shortened their jump down to only 0.015". He mentions this one change shrunk machine rest group sizes by a very substantial 50%.
Whatever chamber you choose I suggest polishing it per instructions in "AR-15 Chamber Selection" (July-September 2014) resulting in significantly better function. If you elect to keep the issued chrome-lined barrel remember the chamber is also lined. Concentrate on a mild "kiss" of the chamber with the polishing hone. You don't want to end up polishing though the chrome lining and into the underlying metal.
Next, land and groove numbers. The military uses six groove barrels but this isn't optimal. They could easily change but refuse to listen to competitive shooters. I guess if a general officer didn't think of it, it ain't worth considering. Derrick Martin and I would be considered old timers by most, having grown up shooting M14s at bullseye targets and earning our Distinguished Rifleman and President's Hundred that way. Occasionally, our range shooting was held up by cease fires called on the line to let dinosaurs pass downrange. During those days the rumor with M14 barrels was odd numbered lands and grooves (three or five groove) shot military ball ammo "more better" than even numbered. I had Boots Obermeyer (obermeyerbarrels.com) make me a 5R for one of my M1As and it was the best .30 caliber barrel I ever owned or tested, bar none! Not only did it shoot match ammo but it also performed well with military-issue "Special Ball", a cartridge loaded with 173 grain FM] bullets designed to improve M14 performance.
Fast forward from Derrick's and my experiences shooting ball ammo in M14s to our quest to improve M16s, it should come as no surprise we tried barrels with odd numbered lands and grooves. Derrick experimented with three groove barrels at the same time I tried five grooves. Both of us found the odd groove barrels shot 5.56mm ball ammo better than six grooves. Today, you can get button-rifled three groove barrel blanks from PacNor and Lilja, and five groove blanks with polygonal rifling from PacNor. Broughton button rifles a five groove barrel blank with what they call a "canted land" similar to 5R designs by other custom makers. Bartlein and Rock will provide 5R cut rifled blanks and Satern will give you a cut rifled drop-in complete AR barrel with 5R rifling and a Wylde chamber. Any of these barrels can be left fat (about 1" diameter) to add weight under the handguards or turned to a skinny A2 contour.
One advantage of three grooves over five is the wider area to locate the gas port hole. With more and narrower grooves, drilling the gas port more likely nicks or cuts a land. Cutting a land with the gas port may not impact accuracy in the long run but it makes break in more difficult and one can significantly damage the barrel when dealing with heavy copper deposits forward of the port when a land is cut or nicked. I cover this in detail in "Gas Gun Barrel Break-In" (October 2014) and recommend reviewing the material. With a quality three or five groove barrel from the makers listed above and a Wylde or shorter chamber you can expect ten shot groups with a decent lot of 55 grain ball approaching one MOA with slightly larger groups with green tip.
Regardless of using chrome-lined barrels or replacing with something more accurate, I recommend thorough cleaning after each range session for optimal performance. All barrels benefit from a proper break in.
Removing Sling Pressure
Shooters gain stability by wrapping their weak arm through a properly adjusted sling for support. A sling helps, and I have used it on the battlefield in combat with both the M14 and the M16 and continue to use it with my Anschutz rifles for squirrel hunting. My squirrel rifles have free floated barrels so I am not applying any pressure to the barrel via sling tension. Rack grade M16s attach the forward sling swivel to the front sight housing and sling pressure is applied directly to the barrel, flexing it. In regular rack-grade M16s the barrel extension also moves inside the receiver, however, we fixed most of that by adding shim stock and Loctite during our improved assembly techniques. Shooters using slings will notice quite an improvement with a properly assembled AR compared to one thrown together per the common military and civilian assembly manuals. Unfortunately, sling pressure still causes some temporary bend to the barrel, influencing zero, especially with optics.
If the customer elected to replace the skinny stock barrel with a full one-incher under the handguards it will resist some bending. The fatter barrel weighs about one pound more and it is a further advantage moving the sling attachment point off the barrel. National Match Course shooters do this with a float tube. This also adds weight and traps in barrel heat but the finished rifle still resembles an issue rifle externally. The added weight might not be welcome in hunting or combat and holding in barrel heat is never a good thing. A plethora of other float tubes have entered the market and many of them are much better ventilated and lighter than NMC units. Certainly these are options that you and your customer may wish to discuss, especially if the customer uses a sling.
Bear with me for just a moment while I climb up on my soap box. I've been associated with military shooting teams since the mid-1980s. During this entire period these teams have been under utilized, underfunded, and misunderstood. Until our wars in the Middle East I was one of the few members of the Reserve Team that had a combat patch. Generals and bean counters tended to think that all the shooting teams did was punch holes in paper and win trophies. When 9/11 hit all that changed real fast, however. Reservists and National Guardsmen from their respective shooting teams stepped forward and volunteered. Some were sent into combat but, fortunately, enough people in charge had the sense to assign most of the shooters as instructors and range cadre at the various Mobilization Stations or Power Projection Platforms.
At these Mob Stations each soldier has to review and demonstrate proficiency in his particular job skills, be it Infantryman, Motor Transport Operator, or Human Resources Specialist. Everyone also has to qualify with his individually assigned weapon, usually an M16/M4.1 have all the respect in the world for Drill Sergeants and was one for many years, but Drill Sergeants have to cover all the tasks taught in Initial Entry (basic) Training from Drill and Ceremonies to Combatives. If your son or daughter was mobilizing, would you want any random Drill Sergeant supervising their marksmanship training or would you rather have an instructor that wore a Distinguished Badge on their chest and a President's Hundred Tab on their sleeve?
When experience in the Gulf demonstrated the need to reach out a little farther on the battlefield, the active component considered all those old Viet Nam-era M14s that had been stored away. However, few personnel knew squat about M14s or shooting at distance because many active duty soldiers put in 20 years to punch the retirement ticket and get out, leaving no institutional knowledge. Reserve component soldiers (Guard and Reserve) tend to stick around longer because the military is supposed to be part time, with some remaining 30 or 40 years. Many on the two reserve shooting teams not only knew the M14 but had earned their Distinguished Rifleman badges with them and were able to show the active component how to do things right. The level of commitment went even further than that. With only 20 or 30 people slotted a military rifle team there wasn't enough to handle the entire workload. Retired shooting team members stepped forward to help out, not unlike civilian competition shooters teaching marksmanship during the two world wars.
As time went on Designated Marksman training grew. The concept has at least one person per squad trained in long range precision shooting, not as a sniper but as a superior marksman. This person could be equipped with an M14 or M16 with ACOG or similar optical sight. After the fact, a number of organizations tried to lay claim to the Designated Marksman program but it was mostly reserve component shooting team members having competed internationally doing the lion's share of the development using lessons learned in those military shooting competitions. When I hear bean counters, politicians, and even other soldiers claiming shooting teams don't contribute it riles me up and should anger you as well. A military rifle team can be completely funded for the cost of one cruise missile! Most of the knowledge and research I write in my articles was learned during my decade-plus tenure as a competitor and armorer with the Army Reserve Marksmanship Program.
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|Title Annotation:||SPECIAL REPORT|
|Date:||Dec 1, 2014|
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