Gas Tube Alignment

We covered gas tubes from the standpoint of function previously in the "Do You Have Gas?" series (January and February 2014.) In this article we will concentrate on the relationship of gas tube alignment to accuracy. Why is gas tube alignment so important? When I shot competitively I started with traditional one handed bullseye pistol, then combat pistol, service rifle, and finally combat disciplines that required application of both rifle and pistol.

I noted early on that many of the lessons learned in one style of shooting applied to others or could be easily adapted to other disciplines. Later in life, when I migrated from shooting to gun plumbing, the habit of looking at what gunsmiths in other fields were doing just came naturally. One of the groups that initially caught my eye were the bolt gunners, primarily long range competitors (Palma shooters and 1000 yard matches), bench rest and some military snipers.

I watched those shooters spending a lot of money on high end actions that were purported to help ensure the bolt face would be square to the axis of the bore and apply equal pressures to the case head for each shot. I actively wondered if any of that could be applied to gas guns. Early in my tour as the Armorer for the US Army Reserve Service Rifle Team one of my shooters had an interesting observation made using his chronograph.

He had been testing several of his National Match AR-15s and noticed the velocity standard deviation of his rifles tightened up noticeably when he shoved rounds up into the chamber to a uniform distance as opposed to just lying them on top of the magazine follower and allowing the bolt to carry them forward into the chamber during slow fire. He wondered if this could also have a positive effect on accuracy. Given what I had observed of the bolt gunners, I readily agreed to investigate.

While testing with the machine rest I made it a point to run ten shot groups first by feeding the rounds "normally" (lying them on top of the magazine follower and letting the bolt carrier push them into the chamber) followed by a second group with the same ammo fed by shoving them up into the chamber before releasing the bolt. I could tell that the hand fed groups were typically better, with the analyzed data indicating a 13.8% improvement in group size from the hand feeding method.

I figured that the more uniform velocity that my shooter had observed accounted, in part, for the reduction in group sizes. It remains my theory that the hand feeding results in more uniform positioning of the cartridge in the chamber and more equal pressures by the bolt face on the cartridge heads. This all conspires to shrink group sizes.

My shooter and I made the results of our combined research known to the other Team members and, to the best of my knowledge, most of the more successful ones still use the technique. Fliers The most common cause of fliers is shooter error. In guns without gas tubes, the second most common cause is usually bad ammo and often this relates to cartridge run out—a condition that occurs when bullets are not seated in line with the cartridge case.

In the case of Stoner platform guns, faulty gas tube alignment may well rival ammo problems for the number two position. Ideally, a gas tube should exit the back of the front sight housing and not touch anything all the way back to where it goes through the delta assembly near the rear of the barrel. The object is for the gas tube to line up perfectly with the bolt carrier key.

If the alignment is less than stellar the entire bolt carrier can be cocked at an angle resulting in the bolt face not being perpendicular to the axis of the bore of the barrel. This situation would result in cartridges being positioned differently in the chamber from shot to shot with differing pressures on the case head by the bolt faces. Very similar to what I had observed previously when I tested the two feeding methods for single shot slow fire.

Gas tube alignment is fairly straightforward in rack grade AR- 15s. National Match Course (NMC) guns built for Service Rifle competition can be a little bit more challenging, however. NMC rifles have to look externally like issue rifles. As competitors and gunsmiths first began to convert from the old M14 design to the Stoner platform, they quickly discovered that the "as issued" practice of attaching the front of the sling to a swivel that was integral with the front sight housing produced really poor results.

To steady themselves, shooters use a loop sling as a brace. If that sling is attached to the barrel of the firearm the shooter actually bends the barrel down and to the side. To rectify the problem smart gunsmiths invented various float or handguard tubes. These

tubes are just a little fatter than the diameter of the barrel and encircle the entire barrel without touching it between the delta ring and the front sight housing. The front of the sling attaches to a sling swivel that is built into the end of the float tube and thus the barrel is not bent. Slightly modified handguards fit over the float tube, making the exterior of the gun look stock.

These tubes are commonly sold as a set which includes the tube itself, the modified handguards and a gas tube that is supposed to be bent correctly for the float tube. In reality, all three of these components are normally made in separate factories. The gas tubes may work on some handguard tubes but they often touch others. Two problems arise when the gas tube is in contact with the float tube.

The contact itself will cause the alignment of the gas tube to suffer and it will enter the bolt carrier key off center, causing the carrier to cock at an angle. The float tube itself looks like apiece of heavy pipe, with wall thicknesses averaging 0.09" and weighing between 1.25 to 1.5 pounds. Looking at them you would swear that they are unbendable by even the strongest shooter. This is not the case, however, and shooters of even modest size who employ a tight loop sling can cause the tube to flex.

As you can easily imagine, any movement in the handguard tube will be transmitted to a gas tube that is in contact with it. This extra movement exacerbates the cocking out of alignment of the bolt face. Fat groups and fliers will result. There are two general sizes of these handguard tubes. One a 1-¼" outside diameter, such as offered by Compass Lake Engineering and Bushmaster. I never experience problems with either brand regarding gas tube to float tube contact.

The more common size is just a little fatter at 1 3/8" outside diameter. Most of these have contact issues. Remember, the gas tubes supplied with these units are not made by the folks that make the float tube body, so gas tubes that easily clear the skinnier 1-¼" handguard tubes will touch the fatter

" ones in most cases. Northern Competition offers one of the better 1-3/8" units. They understand the problems with gas tube to float tube contact and have specified certain bends and dimensions from the supplier they use for gas tubes. Despite this, I still have to perform some minor adjustments to a fair number of their gas tubes.

Visual Checks Gas tube to float tube contact can be found by simply holding the upper assembly with a light source behind it and visually checking for the appearance of light between the float tube and the gas tube for the entire distance between the front sight housing and the delta assembly. Many of the handguard tubes currently on the market have a cut out section right behind the end cap. The gas tube should be approximately centered left to right in that slot.

The gas tube will likely make sort of a relaxed "S" shaped bend as it comes up from that cut out and this is often where contact may occur. This can look very deceptive and may leave you convinced that it isn’t touching very hard. The natural inclination for many of us may be to remove the gas tube and take a Dremel tool to the rear of this cut out and grind a clearance bevel.

While this may successfully remove the high spot from that one specific area, the entire gas tube may then be free to fall down, hitting the top surface of the float tube or at least contacting high spots along its length. More grinding may relieve those high spots only to expose others and you may end up spending a very frustrating half hour grinding away most of the top surface of the handguard tube. A better method involves precise bending of the gas tube.

The point at the back of the front sight housing where the front of the gas tube enters is below the top surface of the float tube. The purpose of that relaxed "S" shaped bend is to simply raise the level of the gas tube to where it clears the top surface of the handguard tube. This can usually be accomplished by simply increasing the existing bends that the gas tube manufacturer added to the tube at the factory. In order to do this with-

As pictured, there is too much torque on the barrel nut. out kinking, breaking, or collapsing the gas tube you will need a tubing bender. The best source for these are automotive supply stores, the same type of fixture used by mechanics to bend brake lines. They are dirt cheap and you will find that in terms of dollars and time spent the enhancement to accuracy makes this a very wise investment.

The tool that I use accommodates a variety of tubing sizes and the one that best fits our AR gas tubes seems to be the smallest size. When I am bending gas tubes I like to leave the front sight housing attached as it helps me grip things and remember which way I need to be bending. I like to hold the handle of the fixture in my weak hand and perform the actual bending with the strong hand gripping the sight housing. I always bend in an upward direction.

First, locate the start of the "S" bend closest to the sight housing. That will be the point where the tube will be bending in an upward direction. When you are starting it sometimes helps to put a black Sharpie mark at this part of the bend. That mark then needs to be positioned on the stationary "bar" section of the tubing fixture.

For right handed operators, the front sight housing will be positioned to your right (with the bayonet lug on the bottom) and the rest of the gas tube will be supported to your left on the circular section of the fixture. Pay attention to keeping the front sight housing perpendicular.

If you tip or tilt the housing much at all when you are making the vertical bend you will also be subtly adding a horizontal component to the bend that will throw the back end of the tube either right or left of the target entry point in the delta assembly. With everything positioned as described you are ready to make the actual trial bend by pulling straight up with the strong hand. You can then check your work by sliding the sight back into position on the barrel.

The back of the gas tube should be above the entry point in the delta assembly but lined up left to right. Pull the sight with attached gas tube back off the front of the firearm and locate the other part of the "S" bend. This will be behind the first bend and will go in the opposite direction to bring the gas tube back parallel with the float tube. If it helps, mark that with a Sharpie also. Position that second Sharpie mark on the bar section of the tubing bender just the way you did before.

This time, however, you are going to rotate the sight 180 degrees so the bayonet lug will be on top and the sight post will be on the bottom. Remembering to keep the sight housing perpendicular, apply the same amount of pressure upward as you did with the first bend. Check your work as before by sliding the sight assembly back on the barrel.

You are hoping that the end of the gas tube will line up perfectly with the hole in the cloverleaf-shaped entry hole in the upper receiver just below the carrying handle or the hole in the delta assembly if one is installed. This seldom happens with first attempts. Chances are that you will be off either horizontally, vertically, or both.

To correct horizontal issues, keep the front sight perpendicular with one hand and grab the back of the gas tube with the other and pull or push in the desired direction past the center point. You will find the gas tube has a lot of "spring back" and will not respond much to a correction. Once you have that lined up perfectly you can work on the vertical component. That will require you to pull the sight back off and apply additional gradual bending either up or down as appropriate.

Avoid the temptation to give up on the bending corrections and cheat by forcing the end of the gas tube toward the hole in the delta assembly as that will simply transfer the misalignment to the short portion of the gas tube, located in the upper receiver, and will cause the bolt carrier to misalign once the bolt carrier key captures the back section of gas tube. Once you are satisfied that the back end of the gas tube is lining up perfectly with the hole in the delta

assembly, push it through and into the cloverleaf hole in the front of the upper receiver. When the rear of the front sight assembly is stopped by the shoulder on the barrel, repeat your previous visual test by holding the upper in front of a light source and checking for contact between the two tubes. If there is the slightest contact go back and add more bend to the tube as appropriate.

I find that it is not unusual for the lock ring at the back of the handguard tube to be just a little fatter than the body of the tube. If that is the case, file a little flat on that surface about 1/4-3/8" wide, using touch up to cover the file marks, to file it down to the level of the tube body. Because we have raised the level of the gas tube upward we are now in danger of having it contact the bottom surface of the upper handguard.

To test for this, remove the both handguards and lay a sheet of paper across the top of the gas tube and replace the top handguard. If you can pull the sheet of paper out without snagging on the top of the gas tube everything is OK. If there is a hang up, note the exact position. Remove the top handguard and mark the spot on the gas tube that is causing the interference with a Sharpie.

Run the bolt carrier forward into battery to give support to the back of the gas tube and help keep the alignment that you worked so hard to establish. With a wood mallet, give the offending high spot a good slap and double check to make sure that doing so hasn’t forced the gas tube down far enough to contact the top of the float tube. Then, try the paper test a second time to see if you have removed the contact between the gas tube and top handguard.

The handguard is tapered, getting skinnier towards the front where it enters the handguard end cap. I have found on some gas tubes, where the manufacturer put the "S" bend too far forward toward the front sight, there is insufficient clearance with regard to the top handguard. In those cases I simply establish a second "S" bend a few inches further back from the original where the handguard is fatter.

Nearing The Home Stretch Just because everything has passed inspection so far that doesn’t guarantee that the gas tube will enter the bolt carrier key absolutely straight. Perform the following final checks. Turn the rifle upside down and look at the one inch or so of gas tube that protrudes through into the receiver itself. Does it look equally spaced in the channel for the charging handle? Slight misalignments may not show up in such a short section.

At this point I always drift out the gas tube retaining pin in the front sight housing which loosens the tube. Keeping the gas collection hole in the bottom of the tube facing down, slide the tube Two targets on the top were shot with a gun with gas tube alignment problems and the ones on the bottom after the alignment has been corrected.

to the rear, about four inches or so. Any misalignment at that point will be much more evident. These misalignments are often caused by the barrel nut not being set at the right torque. Too much torque will tend to bend the gas tube closer to the ejection port side of the rifle. Lighten the torque in about five pound increments and test each time using the gas tube until it is straight for the whole four inches. Too little torque on the nut will cause the gas tube to bend toward the bolt catch side of the rifle.

In that case, increase torque in about five pound increments, testing each time using the gas tube as described previously. Most of the texts recommend that you don’t go below 31 pounds of torque, so keep that in mind while performing these adjustments. When the tube looks centered, move it back forward into the front sight housing and replace its retaining pin. Take the guts (bolt, cam pin, firing pin, etc.) out of the carrier.

The top handguard will need to be off so you can closely watch the gas tube for any movement. Remove the charging handle and insert the naked carrier into the upper receiver. You will be operating the carrier slowly by hand (I recommend the tip of the index finger) while closely watching the gas tube for movement. You will also be closely feeling for any indication that the carrier is encountering much resistance. Many NMC shooters use a Bob Sled magazine for feeding during single shot slow fire stages.

These magazines have no spring or follower to push up against the bottom of the bolt carrier as regular magazines do. To simulate that, I hold the upper in one hand and focus my eye on the gas tube just in front of the clover leaf. As the bolt carrier key comes over the back of the gas tube, watch to see if the gas tube moves either up or down at the front of the receiver. If it does, that indicates a potential alignment problem. Flip the rifle over so it is upside down.

The weight of the carrier will position it closer toward the channel for the charging handle. This is the way the carrier will be when the rifle is fed from a conventional magazine that has a spring and follower pushing up against the bottom of the carrier. As you move the carrier back and forth, watch the gas tube closely as you did before to see if there is up or down movement. If there is, the gas tube is not properly aligned.

If you see movement up toward the carry handle the gun is telling you that the gas tube needs bent up just a little. This can often be done by supporting the upper assembly so it is upside down. Insert a screw driver of about the same width as the charging handle and hold it forward (toward the muzzle) on the gas tube as far as it will go. Give it a modest hit with a hammer. Retest and continue moving the gas tube into alignment.

If you found the gas tube wanting to move down as the carrier key came over the back of it you can use the same screw driver placed between the top of the gas tube and the charging handle channel to pry down on the tube. Keep checking until the vertical movement of the gas tube is cured indicating that the tube is entering the key with no rub or vertical alignment issues. Now it is time to check for left or right movement of the gas tube.

Hold the rifle in its normal position with the carry handle up and run the carrier back and forth as before. This time watch the gas tube for any movement either right or left. You can correct as before by adjusting the barrel nut torque in finer one pound increments or you can use the screwdriver up in the charging handle channel to apply a slight correction. Either method works fine. Performing this whole process will likely take you about an hour the first time you do it and about a half hour with practice.

It is a cheap and effective method of cutting group sizes in any Stoner platform gun that has a gas tube. You may very well discover some guns shrinking groups and eliminating fliers. AG American Gunsmithing Association