Gas Tube Alignment, Revisited

When I published the predecessor to this article way back in March 2014, the title did not contain "Part One" because at the time I thought I had thoroughly covered the topic and put things to rest. Since then, the rules for one of the biggest and most popular matches on the planet (CMP Service Rifle Nationals at Camp Perry) have changed. My original article covered Service Rifle AR-15s as they had to look like a stock M16A2 as seen in any military arms room around the country.

This included A2 look-alike handguards. Hidden underneath, however, you were permitted to install a float tube to remove sling tension from the barrel. Companies offering these units normally marketed them as a set, including the float tube, modified A2 handguards, and a gas tube with custom bends designed to keep it from contacting the handguard at any point.

While the actual float tube may have been made in-house, the modified A2 handguards and custom gas tubes were almost universally outsourced to other specialty houses. This outsourcing resulted in missmatches. Hard contact between the float tube and gas tube threw gas tube alignment out of whack, resulting in fat groups and lots of fliers. My original article dealt with ways to fix this problem, mostly by re-bending the gas tube. It might be helpful to review that article.

Among the more recent rule changes, handguards no longer Middle, handguards removed from a Compass Lake Engineering unit showing the actual float tube underneath. This allows the barrel to float and all sling pressures to be removed from the barrel. The CLE unit comes with a properly-bent gas tube that requires no gunsmith workarounds. Bottom, an example of a quad rail handguard that in some configurations are now legal for Service Rifle shooting in NRA and CMP matches.

Some of these railed handguards come with very fat replacement barrel nuts that force gas tubes way out of alignment. have a M16A2 look-a-like mandate. Both the NRA and the CMP are alike in concerning what now constitutes legal handguards, though there is some difference and a unit that might be acceptable in an NRA match may be disallowed by the CMP. The CMP even changed their wording regarding handguards from one year to the next.

Handguards that were perfectly legal in 2016 were in some cases disallowed in 2017, causing a lot of ire among competitors. Granted, the change was published in the newer rulebook but who would think to even look when the handguard on their rifle passed muster the year prior? These rule changes seemed to be capricious and not grounded in any safety or real-world function issues.

As a shooter or custom gun builder, you now have to consult two different rulebooks before selecting a handguard and read them every year to make sure that changes in word-

The revisited approach more often resorts to removal of metal to correct problems. Far ing or requirements don’t catch you flat footed. And you darn sure better have a fat check book as these different handguards can go all the way to about $400. Pardon me if I seem critical. Both the NRA and the CMP do a wonderful job of running their respective matches and neither outfit gets enough credit for their efforts.

But having two different rulebooks, two different load plans, two different starting positions for rapid fire events, and now two different configurations of what is legal equipment is perplexing and reminds me of how lucky I was in meeting my competitive goals while things were uniform and operated under the KISS Principle. I try to take an optimistic outlook on some of these differences because they can force us to hone different skills that may prove valuable in the real world.

But these differences… Climbing off my soap box and getting back on topic: Even if you area gunsmith who seldom is called upon to work on Service Rifle uppers for National Match Course competitions, the handguards that are so popular in that discipline are also used on Match Rifles, Palma guns, F-Class, 3 Gun, certain metal impact target sports, and tactical builds. In fact, ARs for some of these disciplines have been sporting these handguards before the NMC crowd. What’s The Big Deal?

Before we discuss how to diagnose and fix gas tube alignment issues in these new handguards, it is important to understand why gas tube alignment is so important to achieving tight groups. It all relates to precisely aligning the bullet to the center axis of the bore. Errors in achieving this stellar alignment result in bullets entering the bore at an angle and deforming. Thousandths of an inch area big deal in this bullet alignment business.

Actors that have a vote in getting bullets properly aligned are chamber dimensions ("AR-15 Chamber Selection" in the July-September 2014 issues), how square the receiver face is in relation to the barrel extension (watch for an article on this soon), cartridge considerations ("Better Ammunition", April 2016), and bolt carrier matters ("AR-15 Upper Quick Fixes: Match Bolt Carriers", May 2014 and "Combating AR-15 Bolt Carrier Tilt", May 2017).

Improper gas tube alignment causes the bolt carrier assembly to cock at an angle when the carrier key goes over the back of the gas tube up inside the receiver. The bolt face needs to be perpendicular to the center axis of the bore in order to center bullets and this can’t happen if gas tubes are positioned at an angle. Test target results prove this.

Diagnostics And Assembly When I am in the process of building an AR, regardless of the discipline, I always use the following procedure and it might be helpful if you adopt the same steps. I insert the barrel extension into the upper receiver. Take the front sight assembly or gas block with gas tube attached and begin sliding everything down the barrel while watching the back of the gas tube as it draws closer to the cloverleaf-shaped hole in the upper receiver.

The tube should enter this hole straight on and without any assistance. If it fails to do so, perform whatever judicious bending that is required to get it to comply. I suggest referring to my previous article on this for tips on doing it right. This needs to be as perfect as possible. With this done, pull the gas tube off and install whatever serves as a barrel nut as provided by the handguard manufacturer. There are numerous styles and this part is where trouble often boils just under the

The bolt carrier will be tilted at an angle, causing rounds in the chamber to be out of alignment with the bore and resulting in deformed bullets. Bottom, a properly-aligned gas tube and bolt carrier will result in smaller groups and fewer fliers. surface. You may have a nut that allows a torque wrench to be attached. The average sweet spot torque for 20" heavy Service Rifle barrels that area full inch under the handguard is 47 foot-pounds. Skinny A2 contour barrels take 60 ft-lbs.

Neither setting is likely to be optimal for your particular rifle but they are good starting points if you plan to tune barrel harmonics for best groups. See "AR- 15 Barrel Harmonics" (October 2016) for tips on how to perform harmonic tuning. Done right, harmonic tuning is usually good for about an average 20% reduction in group size but it does require actual range firing of groups and is fairly time consuming.

Once what serves as a barrel nut is in place, slide the gas block with gas tube attached back down the barrel, making sure that nothing is interfering with it going effortlessly through the clover leaf. If there are issues, correct them. Here I perform several tests. Although there is flexibility in the order, don’t skip any of them. Hold the rifle up to the light and make sure there is a distinct line of white between the top of the barrel and the bottom of the gas tube, just as Stoner originally designed.

Pay particular attention to the barrel nut. Some of these are manufactured too thick and can jack the gas tube up at an angle. With the gas block and tube pulled off, look through the back of the receiver at the cloverleaf hole and make sure there are no visible obstructions on any of the four corners. Some of these barrel nuts are so oversized they look like humpback whales sticking up in the bottom of the cloverleaf. Failure to properly correct that will guarantee fat groups and lots of fliers!

If there are any teeth or holes in the barrel nut through which the gas tube must pass, don’t forget this last test: Run any straight gas tube with bends similar to the one designated for your rifle into the cloverleaf from the front. Keep the gas hole pointed down. Continue to push the tube further into the receiver until the back of the tube is about even with the back of the receiver.

This technique allows seeing misalignments that wouldn’t be as apparent when the gas tube is in normal position with only an inch or so showing up inside the front of the receiver in the channel cut out for the charging handle. If the gas tube points towards the ejection port side, the torque is too heavy and needs to be reset by a foot-pound or two lighter Re-torque and inspect again. On the other hand, if the gas tube points toward the bolt catch side, the torque is too light.

Reset for heavier torque and try again. There is one correct setting on issue barrel nuts and a lousy foot-pound on either side of that setting will have the gas tube pointing left or right of center. I like to hit the gap between the teeth where the gas tube passes a lick with a Dremel tool and widen it out just a little so I have insurance against misalignment.

Corrective Measures Concerning minor left to right errors in alignment caused by improper torque settings, looking through the back of the receiver at the cloverleaf you’ll notice either George or Gracie (the humpback whales in Star Trek IV: The Voyage Home) sticking up and obscuring free insertion of the gas tube. The only viable recourse is removing serious metal! Don’t try fixing this by re-bending of the gas tube as we did in the previous article.

Instead, start by running the gas tube through the cloverleaf and then marking each side of it with a white Sharpie on the top surface of the offending barrel nut. This leaves a set of railroad tracks about 1/4" apart and shows where metal needs to be removed. Spin the barrel nut free of the receiver and go to work. If you area professional gunsmith with a fully-stocked shop, you don’t need me to tell you how to proceed.

Some of these barrel nuts have three sections with the front section closest to the muzzle being thinner than the two really thick rear sections. You want a channel through the thick sections that is not less than 1/4" wide but don’t cut through the bottom! The bottom thickness should be about 0.083" which mimics the thickness of a regular barrel nut as Stoner originally designed. You may ask if this isn’t causing the replacement barrel nut to be weak.

I work on ARs every day and have never encountered a barrel nut failure. There was absolutely no safety reason for handguard manufacturers to make their replacement

nuts so thick! I suggesting chucking these overly-thick replacement nuts in a lathe and turn the circumference down to the 0.083" dimensions of a stock barrel nut. The handguard portion slips over the barrel nut snug as it comes from the factory and is then clamped in place by various screws and fixtures.

Turning down the whole circumference of the replacement barrel nut will make it impossible to mount the handguard section and you will end up groveling to the manufacturer for anew one because you screwed up their screwed-up nut that they provided with the handguard kit. This is a stack up of screw ups rivaling the height a plate of pancakes at Denny’s! Cutting a channel or filing a flat on just the top of the barrel nut will not degrade the further mounting of the actual handguard section one iota.

It will be covered by the handguard section and completely invisible. If you area hobbyist, your options will be more limited. Most will grab a big file and start working on a flat. This works but is time consuming because there’s a lot of metal to remove. Some of these replacement custom nuts I have measured as large as 0.176" thick, 0.094" thicker than the original, so that’s nearly a 1/10 of an inch. See you in an hour or so! Careful use of a Dremel or a grinder can shorten the pain of this task significantly.

Neatness doesn’t count; function does here. If you don’t want to invest the time and considerable effort doing it yourself, take the thing to a machine shop along with copious notes from this article and paythem to do it for you. It sucks to have to pay someone to fix the manufacturer’s screw up after already paying $400 for the original purchase but there is a cost of doing business and often a greater cost for trying to look cool on the range with a fancy handguard that has rails on every possible side.

Adding additional insult to injury, in spite of paying big bucks for all those rails, jacking up the final cost by a bunch, a lot of match rules prohibit attaching anything to them! I find rails on the sides, and especially on the bottom, to be uncomfortable on my support hand even with a heavy leather shooting glove. Perhaps I need to start taking a testosterone supplement? Regardless of what exact method of metal removal you employ, inspect to make sure you have removed enough.

If you end up with a flat, hold everything up to the light and make sure you can see daylight between the top of the finished flat and the bottom of the gas tube. If you end up with a channel, it will be harder to make a visual determination. Tear apiece of paper about 3" long and 1/2" wide and slide it in the channel under the gas tube, running it back and forth the length of the cut. If you have any hang ups or snags, remove a little more metal. If the paper moves freely, the machining was adequate.

Lest you think that all the fun has ended, you are probably wrong. Due to manufacturing tolerances in the barrel extensions from one lot to another, the next time you rebarrel the upper it is very likely that your flat or milled channel won’t time up right at 12 o’clock and you will have to start all over again. These replacement barrel nuts are like the gift that keeps on giving but in a very aggravating way!

Far Actual metal removal, such as the aggressive flats filed on the top of the one pictured here, will be required to fix the problem. This is a JP Enterprises handguard that is done right and doesn’t require any gunsmith workarounds. Why In The First Place? At this point you are likely asking yourselves why the manufacturers of some of these railed handguards created them with these over-sized replacement barrel nuts, thus making life difficult for gun owners and gunsmiths.

I see evidence that some sound reasoning was involved. Because of the way that the body of the handguard attaches to the barrel nut, the outside diameter of the nut becomes the inside diameter of the handguard. So a fatter barrel nut results in a fatter cavity inside the handguard and greater clearance from the barrel that is housed inside of it. I rather imagine this was an overriding consideration in the design of these units. In order to shrink the O.

D. of the barrel nut, the inside cavity of the actual handguard would also shrink, providing less clearance to the barrel that lives inside of it. Unfortunately, many makers of custom handguards only manufacture this one part and don’t build full custom guns. They are applying tunnel vision and remain oblivious to the impacts to other parts of the gun they have inadvertently created.

Alternatives There are other very good alternative handguards on the market which result in floated barrels and require zero workarounds. The Compass Lake Engineering 1 1/4" float tube comes with the tube, modified A2 (G. I. looka-like) outer handguards, and a prop-

erly-bent gas tube that won’t require any adjustments. The whole set up costs about 1/3 that of the overpriced quad rail units. If that is too much of an old school look for you or some of your customers, I can recommend the JP Enterprises Modular Hand Guard System. These are available in a variety of lengths for building everything from tactical carbines to Match Rifle, Service Rifle, and Palma ARs. It is round and, like the CLE unit, very comfortable to the support hand even without a heavy shooting glove.

If you are building a gun for CMP and/or NRA Service Rifle matches, thoroughly research the current rules to ensure that it is legal. I often write directly to CMP and attach a picture if there is a question on a particular handguard or other part. Many shooters were burned in 2017 and I want acknowledgment in writing that I can provide my customer to include with their range kit.

If you or your customers are simply in love with rails (they are the current flavor of the week, after all) JP Modular units are drilled and tapped on all four sides and the company has rail section accessories in various lengths. The CMP 2017 rules were so draconian that even the percentage of the handguards that are covered with rails was dictated. What possible sense this all makes still baffles me and many shooters. If attaching accessory rails, be careful of the screw lengths and positioning.

I’ve seen the gas tube extending up close to the inside of the handguard in certain areas and you don’t want screws extending down and contacting the gas tube. Both JP and CLE build whole ARs for competition as used by some of the biggest names in competitive shooting, so both companies understand the entire gun. I prefer to buy parts from them because they don’t result in unintended consequences elsewhere in the gun.

Real World Examples I was lucky, or maybe I just followed good shop assembly and inspection procedures that only take minutes to perform. I identified and corrected found problems in the first of these quad rails to make its way to my shop.

Others were not so lucky and I soon started hearing from them either by personal visits to me at Camp Perry (Commercial Row Building 910-A during the CMP Service Rifle Nationals in late July) or bye-mail inquiry The folks that I heard from were individuals who had other types of handguards on their rifles initially and knew the accuracy potential of their guns. They had taken those original handguards off and substituted the stylish quad rail units, only to have their groups go to hell in a handbasket.

I was glad to share the information in this article with them on how to restore their accuracy. About half of them opted to perform the work themselves and the others just handed me their upper or shipped it to me to perform the corrections. I tried to keep in touch with these folks as much as possible and every one of them that I was able to get feedback from said that after the accuracy-robbing gas tube alignment problems were corrected their original accuracy came right back!

What worries me are the guys who are doing brand new builds using these quad rails. I guarantee you their accuracy is being degraded and most of them have no clue as to the culprit. As is nearly universally the case, I predict they are blaming the barrel maker because they nearly always get the blame. Or they are blaming the gunsmith that did the work. Unfortunately, many gunsmiths are unaware of these problems and are misplacing their trust in the handguard manufacturers to do it right.

That’s what motivated me to write this article. Part 3? I dearly hope I won’t need to submit a Part 3 on gas tube alignment in a couple of years. It does seem to me, however, that just as fast as I come up with cures to such a problem, good ol’ American ingenuity invents new ways to throw monkey wrenches into the mix. On top of that, it seems new companies are springing up very rapidly, with each trying to outdo the others in the business in trying to score points on "cool" designs with less emphasis on accuracy.

I guess I should look at all of this as job security. As long as companies insist on screwing up their designs, I will have plenty to write about. There are numerous handguards out there I don’t yet have experience with. I am sure there are issues with some of them but I hope that if you encounter these my articles will help diagnose and correct any gas tube problems you may encounter.

I mentioned earlier that shooters discovered on their own by shooting normally off the shoulder that their groups had gotten fatter and that fliers had increased when they substituted new railed handguards in place of older methods of floating the barrel. After the gas tube alignment issues were corrected they saw their accuracy performance restored. I have also verified this using a machine rest.

Some of those folks, meaning well, have reached out to those handguard manufacturers in an attempt to call their attention to design flaws. As a professional gunsmith I have done this as well. It is a real tightrope to walk. Nobody likes getting an "F" on their homework! I am still of the opinion that Penn State gave me a degree mostly just to get me off campus and make room for smarter students, so I understand. Unfortunately, some of these manufacturers responded with condescending attitudes.

I suppose this depends on which tech person you talk to on a given day, the way you present your case, and what kind of day the person is having. Between the attitude at some of the factories and the difficulty of a total redesign of the whole unit I mentioned previously, I strongly suspect that we professional gunsmiths are going to have our work cut out for us for quite awhile. AG