Combating AR-15 Bolt Carrier Tilt

Eugene Stoner never designed his AR-15 to be a tack driver. It was intended to be a lightweight battlefield weapon but by good fortune it happens to be easier to work on than its predecessors. In spite of certain design elements, it is far more accurate than it has any right to be. With just a little bit of work, ARs can be made to perform nearly as accurately as expensive custom bolt rifles.

In order to shoot tight groups with any Stoner platform, the custom gunsmith has to correct certain elements of the rifle’s design. Among them are tightening the fit of the upper to the lower and making the barrel extension more rigidly stabilized in the upper receiver. Getting uniform bolt lock up and consistently The large (0.078") gap shown at the top of the carrier hints at the severity of the problem. Use feeler gauges to measure. positioning the chambered cartridge is next.

I have written a number of detailed articles for American Gunsmith covering this and more. In order to fully integrate the material that follows here, it would be prudent to review this. For example, it makes little sense to correct bolt carrier tilt if it just throws the gas tube out of alignment.

For those of you occasionally building a Stoner rifle for single round feeding only, review information and options on neck dimensions concerning chamber selection and conduct magazine checks on precision rifles feeding normally. Although I and other gunsmiths have known about how bolt carrier tilt degrades accuracy in Stoner platforms for decades, the first time I mentioned it in this magazine was near the end of "AR-15 Upper Quick Fixes: Match Bolt Carriers" in the May 2014 issue.

I have talked to gunsmiths and industry folks who actually knew Stoner that were aware of this issue when my younger self was running around the jungles of Viet Nam with an early XM16E1. Why is bolt carrier tilt a problem? The "bore" of the upper receiver is the hole where the carrier assembly lives. It’s fat to promote reliable function but that also results in bolt carrier tilt due to gravity. When at rest, the tail section of the carrier bottoms out on the rear of the receiver.

Since the bolt is housed in the carrier, its face tilts at an angle to the bore of the barrel when the bolt face should be perpendicular to the center axis of the barrel. Gunsmiths building custom bolt action rifles use tight, expensive, custom actions to achieve this, some costing more then entire AR-15s. Bolt actions can have tighter tolerances due to manual action but self loaders can’t.

Some AR shooters insert weights into the back of the carrier to increase dwell time (how long the bolt stays locked up upon firing) which makes tilting worse. You can see bolt carrier tilt by shotgunning AR and observing the carrier’s rear in the upper receiver. This is how things line when using a Bob Sled or other solid, single feed magazine with no follower spring.

The trough facilitates single hand feeding and a spring-loaded plunger engages the bolt stop, but no spring exerts upward pressure on the bottom of the bolt carrier to cause carrier tilt. Conventional magazines complicate carrier tilt as their follower, spring, and ammunition exert upward pressure. Manufacturing tolerances, wear, damage, dirt, etc. causes

individual magazines to exert slightly different upward pressure. To see this, clear an AR-15 and slowly insert a filled magazine with the ejection port open. You’ll see the carrier through the ejection port rise up a bit (about 0.010-0.020") as the magazine locks in place. The top cartridge pushes the bottom of the carrier up. The amount of upward lift depends on the fit of the upper to the lower, characteristics of the magazine, and/or dimensions of the upper receiver "bore" and bolt carrier.

A true match bolt carrier that is fatter in the front will normally exhibit less lift. This demonstrates what happens to the front half of the bolt carrier observable through the ejection port, but this article is about bolt carrier tilt and that happens to the back half of the carrier invisible when the firearm is fully assembled.

To visualize this, assemble an AR- 15 with the buffer tube (some manufacturers refer to this as a "lower receiver extension") and the stock installed last so the carrier is observable through the back threaded part of the lower receiver. Inserting a magazine then reveals how much tilt is experienced at the rear of the carrier.

If you aren’t assembling an AR-15, take a complete upper half and hold a filled magazine below the carrier where the lower normally would be until the bottom of the carrier is pushed up by the top round. It is easy to "overstate" the amount of lift using this method, however, so stop pushing upward before the feed lips contact the bottom of the carrier as this is more upward pressure than found in an assembled rifle as the increased friction would induce stoppages.

Before you ask, the upward lift from a magazines is not a "self fix" correcting bolt carrier tilt. Because of the magazine variables described, no two magazines lift the carrier exactly the same and such variables conspire to alter point of impact. Consider that a car that doesn’t sit level while on flat ground will still be crooked if raised by a mechanic’s lift. The same is true with bolt carriers. Far Bob Sled magazines for the AR-15, AR- 10 (for you grownups), and .22 L.

R. (far right) exert no pressures on bolt carriers. There was an understanding of this issue early in the AR-15’s development. I have been lucky enough to find a few of these guys still around who told me of their early efforts. Initial attempts included silver soldering strips of thick shim stock to either side of the carrier bottom. These sections of raised shim were called "anti-tilt pads" and the term has stuck, though the approach hasn’t. Groups were improved but function was hampered.

Consider a firearm where the buffer tube isn’t in perfect alignment with the bore in the upper receiver. That could cause those silver soldered anti-tilt pads to crash against the leading edge of the tube, much like a potato peeler turning up a section of the pad and jamming up the whole gun.

With early efforts not being entirely satisfactory, it seemed like most gunsmiths just ignored the bolt carrier tilt problem for decades, though I kept it in the back of my mind hoping for "divine inspiration" or anew "smart guy" that might arrive at satisfactory solution. I had my gunsmithing hands full keeping nearly 300 AR-15 rifles operating and researching other improvements to U. S. Army Reserve Shooting Team firearms and was swamped with work anyway.

I remembered reports from early gunsmiths silver soldering shim stock on carriers caused function problems. My initial goal was to try to find a low tech, affordable method to level the carrier, and then run enough machine rest tests to see if accuracy was enhanced enough to warrant further research. The secondary goal was reliability. In

my "misspent youth" I competed in a number of pistol competitions, earning two of my three gold Distinguished badges that way. I own several 1911 pistols customized by Jim Clark, Sr. Some of them have a Giles Slide Guide, a built-up section at the front of the dust cover. The guide is drilled and tapped for two set screws set at 4:30 and 7:30 o’clock, each advanced equally inward until they bear against the bottom of the slide to remove vertical and horizontal slop.

Machine rest groups proved this effective in tightening groups. My first anti-tilt pads borrowed this idea. Not all that difficult! Begin by drilling two holes in the upper receiver, one on either side tight against the lug for the rear takedown pin, and tap to 8-32—the same thread as used in most windage-adjustable front sight housings with four set screws. Use 1/4" long Allen screws. Employ feeler gauges to measure the gap between the top of the carrier and the receiver.

Run each set screw up to where they just touch the bottom of the carrier’s rear. Cautiously continue to move each set screw equally while taking periodic feeler gauge readings. When the gap between the top of the carrier and the receiver is halved, it should be level. Double check with feeler gauges to ensure there is the same measured gap between the bottom of the carrier and receiver.

Once set, thoroughly degrease, then douse the female threads in the re- Far Allen screws are moved in about four times higher than they would be normally to show up better in the photo. ceiver and the threads on each set screw with red Loctite and repeat one last time. To avoid malfunctions, mill, grind, or file a 45-degree bevel on the abrupt factory 90-degree leading edge of the carrier’s bottom rear.

I also heavily grease the tops of the set screws and corresponding bottom section of the carrier with the Tetra Gun Grease. Thoroughly clean the carrier and receiver after each range session. An ultrasonic unit is helpful. I have never had a malfunction to date with these pads, however, the very first time I attempted to clean the bore of a rifle with this first generation design I discovered I couldn’t get my bore guide in the upper receiver because the set screws protruded too far into the receiver.

The fix is to mill clearance slots in the bore guide but the cost of a quality metal bore guide plus machining time could easily be greater than the cost of installing the anti-tilt pads. Because I wanted the fix to be affordable, I went back to the drawing board. Far For first generation pads, (left) heavily coat the tops of both set screws inside the upper receiver and corresponding contact surfaces on the bottom of the carrier tail.

Right, heavily coat generation two pads and the corresponding surfaces inside the receiver.

An ultrasonic unit will be helpful. Before I describe my second generation attempt, I’ll tell you an interesting story with the first generation pads. In my December 2015 article "Magazine Checks", I showed a target scan where one magazine shifted point of impact 2.25" from two other mags at 100 yards and another demonstrating a shift at 300 yards. Those shifts were dwarfed by something that happened when first testing my generation one anti-tilt pads.

I initially had not added Loctite to the threads of the set screws and was beginning my machine rest test groups. One of the two set screws moved of its own volition, likely due to vibration These test groups show how far a magazine can move the downrange POI at 100 yards. or forces caused by rubbing against the bottom of a moving bolt carrier. I found out later that it was actually screwing up, putting increased lift to the back of the carrier with each shot.

After 18 shots I could tell that the carrier was running funky and was near the point of locking up. Those 18 shots began at 6 o’clock and tracked an incredible 6.75" up and off toward 11 o’clock, way more than I would have ever expected. I meant to save the target so I could scan it and include with this article but somehow it must have mistakenly gotten discarded.

Sorry about that, however, this clearly points to the fact that the orientation of the bolt carrier has an enormous influence on point of impact and their need to run straight. We also need them to operate consistently or our groups and zeroes will suffer. For my second generation design, I returned to the decades-old practice of locating the pads on the bolt carrier tail using 5-40 (3/16" long) cap screws with small button heads about 2/10" in diameter.

The small heads contribute less drag but provide the needed support. The carrier tail is drilled, tapped, and counter bored at both the 4:30 and 7:30 positions. The locations provide upward lift as well as a little side-to-side stability. The counter boring permits the heads to seat flush with the sides of the carrier in the event the pads interfere with function in any way.

I like the domed-shaped heads because their taper gently eases the carrier both as the back of it enters the buffer tube and travels rear, and then a second time on the forward motion as it reenters the back of the upper receiver. Gunsmiths should thoroughly inspect the area where the Allen wrench enters the top of the head for any burrs or imperfections and stone these absolutely smooth. The adjustment of the cap screws is similar to the first generation.

Advance both screws outward in equal amounts while taking feeler gauge readings. When the gap between the carrier and the receiver at the bottom equals the gap at the top, the carrier should be level and perpendicular to the bolt face and bore axis. As before, generous amounts of red Loctite on the threads holds the adjustment. Lube the button heads liberally with Tetra Gun Grease. Always thoroughly clean bolt carriers following every range session and clean with an ultrasonic unit if possible.

Don’t be afraid to add lube to bolt rings and carrier antitilt pads during extended range sessions. I have not had a single malfunction yet with either generation of anti-tilt pads.

Candidate Firearms Thoroughly inspect customer firearms before undertaking a job to note any potential to compromise function or add drag. You can usually see outwardly any misalignment between the upper and lower receivers. As long as both receivers are of the same brand there is Skeletonized carrier on far left will not work with either pad design. usually no problem. Mixing and matching brands was historically more problematic.

Today, there are only a few "chip makers" actually making receivers from scratch so receivers from different companies may be made in the same foundry and stamped with different names. Uniformity and quality has gotten better in recent years. If in doubt, remove the hammer, buffer, and spring from the lower receiver. Gut the bolt carrier by removing the bolt, cam, firing pin, and retaining pin. Place the carrier back in the upper receiver and close up the weapon.

Holding the firearm horizontal, tilt it so the muzzle is higher than the stock and the stripped carrier should travel to the rear of its own weight. When the motion is reversed, the carrier should return to its normal forward position. Flip the rifle upside down and repeat the test. If there are no pauses or hesitations, install the anti-tilt pads of choice and repeat the experiment.

Obviously, any Accu-Wedges, Accu- Screws, or other tensioning devices Far Cap screw is positioned deliberately higher than normal to show up better in photo. Add grease to anti-tilt pads for the same reason. designed to tighten the fit need to be in place during these tests. Always admonish the customer to keep the carrier clean and well lubed and to thoroughly function fire any rifle that has undergone any work before leaving on an important hunt or competing in a match.

Also caution the customer to expect some sort of a shift in zero as a result of your work. Unfortunately, not all bolt carriers are candidates anti-tilt pad modifications. Consider a carrier that extends metal forward far enough to operate the trip sear for full auto compared to those that don’t. Those cut back too far don’t have enough metal surface for pads. Such a carrier is only good for those instances of rifles not getting enough gas and the lighter mass can correct some function problems.

My advice is to equip every rifle you build with a true match bolt carrier from Les Baer, JP Enterprises, or Young Manufacturing. Just dropping in a true match carrier can cut your group size an average of 8.8% even without anti-tilt pads. Expectations I have run numerous tests of both generations of pads and keep coming up with a figure of 14% average reduction in group size with both approaches.

That is a nice accuracy gain especially when it is combined with the 8.8% average improvement of just dropping in a true match carrier. Make absolutely sure to check gas tube alignment after leveling the carrier! You may have to make some minor tweaks.

The left end of the buffer is sized to just fit into the back of the carrier, lightly "locking" the two together. I don’t know what to expect with regard to longevity when employing the first generation set screws. The twin set screws in my Clark 1911s with Giles Slide Guide have remained straight and tight after tens of thousands of rounds, however, they are drilled and tapped into steel. The generation one twin set screws are drilled and tapped into less substantial aluminum. Will they hold? I am not sure.

Accurized M9/Beretta M92 pistols use set screws in their aluminum frames and have worked fairly well.

The second generation approach seems to counter side-toside movement of the carrier’s rear a little more effectively, but machine rest tests prove both methods equal in reducing group size, indicating that up and down slop in the carrier has more influence on group American Gunsmith Mailing List Policy Like many other publishers, we make portions of our customer list available to carefully screened companies that offer products and services we believe you may enjoy.

If you do not wish to receive these offers, and/or information, please let us know by contacting us at: American Gunsmith Opt-Out Program Norwalk, CT 06854 Please include your current mailing label. size than horizontal movement. That ’ s fine, because it was downward tilt that we struck out to correct in the first place. Note, avoid the temptation to get greedy and over-adjust anti-tilt pads. These simply level the carrier so the bolt face becomes perpendicular to the center axis of the bore.

Overadjustment results in tipping the rear of the carrier higher than the front and ruins accuracy as surely as letting the back of the carriers continue to tilt downward. What Didn’t Work The February 2013 issue of the NRA’s American Rifleman featured an article ("Piston Delivery") on the Adcor Defense B. E. A. R., an external piston AR. These were flavor of the month then, but the interesting part was the company created anti-tilt pads into their bolt carrier.

They appeared to be integral and machined with the rest of the carrier as opposed to being clapped on afterwards. It wasn’t clear to me from the article if their purpose was to enhance accuracy or function. By then, I was gunsmithing on my own instead of for the Army Reserves and decided to test these at my own expense. External piston AR-15s have brought bolt carrier tilt back into the conversation after decades of ignoring the issue.

I found a heavy custom buffer that was designed for external piston guns and purported to eliminate carrier tilt. The section to the extreme left fits into the back of the carrier. This modified buffer was supposed to improve function in external piston guns and did not promise any accuracy improvement. I was hoping that it might and set up a study to determine if it worked. I was also hoping for a tilt solution in carriers with short bottom sections that don’t work anti-tilt pads.

I tested a good cross section of AR- 15s with stock carriers and then a second time by substituting the heavier buffer for the stock one. There was no significant improvement and no degradation. I determined that to get any accuracy improvement, the fattest part near the front needed to be significantly larger in diameter than manufactured. Unfortunately, not everything we try ends up working and this is just another example.

In fact, I wrote a dedicated article on this called, "AR- 15 Build Problems" in the May 2016 issue. Many positive results are "baby steps" arrived at by trying and learning new things over time. I make no claim to having completely solved a problem that has been around longer than most of you readers but I hope to have convinced you that carrier tilt does limit accuracy and by a greater degree than many have suspected. There are ways of dealing with the problem.

Most of all, I hope some of you smart guys will deliberate and experiment to come up with better solutions than the tiny baby steps I have taken. Nothing would make me happier than to open the magazine in the near future and discover an article written by one of you detailing a better method of correcting bolt carrier. Please, have at it with my blessings! AG