AR-15 Diagnostics

There are good reasons why we want the bolt to lock to the rear when the magazine has been run dry. It is a quick indicator of being out of ammo and can make reloading a bit faster. While some have advocated counting rounds, the reality is most people won’t in a fight. Military personnel sometimes use tracer rounds to indicate the magazine is almost empty but this isn’t a viable solution in many situations. It’s best to have the firearm work as intended.

So you’re not wasting time, some Stoner platform firearms don’t normally lock back on the final round. Pistol caliber and rimfire conversions usually don’t. However, it’s possible for an AR-15 that’s supposed to lock back on the last round function perfectly with a full magazine and fail to do so. Understanding the cycle of function is key to diagnosing such problems. One of the best resources about this is Army FM 23-9 M16A1, M16A2/3, M16A4 Rifle Marksmanship.

What we gunsmiths have to grasp is the difference in the bolt carrier travel necessary to reliably feed rounds from the magazine compared to the significantly longer travel required for the bolt catch to lock the bolt back on the last shot. This is because the flat surface of the bolt catch that traps the front of the bolt is located a fat 3/8" to the rear of the magazine. Anything that interferes with the carrier traveling that extra 3/8" can cause a failure to lock back.

I can think of five primary causes of failures to lock back and an additional seven occasional causes. First, get some information from your customer. Has lock back failure always been a problem with this rifle or is it something that surfaced recently? Has he replaced any parts (including magazines) or changed ammo when this started occurring? Were there any other problems noted? What may seem like two unrelated problems could be from the same cause.

I once had a customer whose fancy two stage trigger had an adjustment screw back out and allow a small captive coil spring to escape. She lost the second stage of her trigger pull and at the same time her hammer started failing to recock a significant amount of the time. The gun would also not lock back on the last round of a magazine. When I disassembled her lower I found the boogered spring trapped right under the hammer and adding so much drag there that it was difficult to cock.

That stiffness prevented the bolt carrier from its full rear travel. Once the spring was removed and replaced everything went back to normal. Consider maintenance and lubrication. In my article, "Do You Have Gas?" (January and February 2014) I established that most malfunctions are due to inadequate maintenance, bad ammo, or magazines. Failures to lock back are no exception. Obviously, if the bolt carrier, chamber, and upper receiver area housing moving parts is dirty the increased friction will inhibit function.

It never hurts to thumb cock the hammer and gently lower it a few times to check for undue resistance there. Inspect the chamber, preferably with a bore scope. If the chamber looks rough or has corrosion, polish it. See details in "AR-15 Chamber Selection" (July-September 2014.) Admonish your customers to use a chamber brush at the end of normal bore cleaning and use a corrosion inhibitor in the bore, chamber, and muzzle crown any time the firearm is going to be stored for more than a handful of days.

The bolt carrier assembly needs to be cleaned and lubed after each range session. Lube can be added to the rings during range fires to keep everything functioning properly. The final inspection is of the bolt catch. Make sure that it moves up and down freely with no drag. This is an area that is often neglected during cleaning and crud easily builds there. Aerosol gun scrubbing compound and a parts brush quickly cleans it.

I don’t recommend that customers disassemble this, however, should you feel any drag remove the roll pin with the appropriate 3/32" punch and inspect for burrs or cracks. After a thorough cleaning and reinstallation, if there’s still drag, replace the bolt catch, plunger, spring, and roll pin. This catch is typically damaged if the hammer falls forward with the upper receiver off. Next, consider ammunition. Weak ammo can fail to generate enough gas to fully function the rifle.

Try test firing some of the customer-supplied ammo in his gun with his magazines, then use a known-reliable load. If your ammo works and his doesn’t find out what his load is and look it up in a reloading manual. Examine his casings for signs of pressure. If it appears that he is using enough powder but his reloads still aren’t functioning ask if he is bumping his shoulders back the required 0.003" necessary for proper function.

If he gives you a blank stare or just says he sizes his brass but doesn’t know how to adjust his sizing die for shoulder bump you

Far 45 ACP Olympic Arms unit, don’t lock the bolt back on the last shot. 22 LR conversions, such as this M261, do not lock the bolt back on the final shot. may just have found the problem. Casings have to be trimmed from time to time. Failure to do so can result in them wanting to stick in the chamber upon extraction. Measure the full length of some of the customer’s brass and compare to what the loading manuals call for with regard to maximum length.

Too-light ammo can fail to produce enough gas and too-hot ammo can also be a problem because the expanded casings may pose an extraction problem. Look at the condition of the customer’s fired primers for signs of pressure and inquire about powder charges. Powder charges are relative to On right, the bolt carrier must travel about 3/8" further to the rear to achieve lock back. barrel characteristics and chamber dimensions and a Stoney Point reading may be needed to measure chamber length.

Generally speaking, chambers with longer free bore pressure up less than ones with shorter dimensions. Foreign military 5.56mm ammo will probably fit AR-15 chambers and fire the first shot but the ability to feed and fire subsequent shots with some of this stuff is anybody’s guess. Many foreign military weapons have gas ports located at varying distances from the chamber and this has a massive influence on gas port pressures and how weapons function with the ammo designed for them.

Not only is the location of the gas port often different, the size varies as well. Also, many foreign rifles use external piston systems that differ from the internal piston design typically found in Stoner guns. The ammo for these foreign guns was designed to function them optimally but the ammo characterizes are often at significant odds in our rifles function wise.

So, if you learn that your customer is having failures to lock back or other function issues with foreign military ammo be sure to try the gun and magazine using a known lot of U. S. manufactured ammo before going much further. On to magazines. In order to diagnose magazine-induced failures we need to understand how lock back is accomplished when everything is operating properly. At the extreme rear of the magazine follower is a "shelf" a little over 1/4" wide and just shy of 1/8" deep.

When there is ammo in the magazine the follower is buried beneath it down inside the body of the mag. When the last shot has been fired the follower is free to move up and that shelf engages the extreme front surface of the bolt catch. At rest the bolt catch is normally held in the down position by its spring and plunger.

When the stronger magazine spring pushes upward on the follower the weaker bolt catch spring is overcome and the bolt catch pivots upward about 1/4" in front of the bolt, stopping and holding it to the rear. When the bolt is held to the rear and the operator depresses the pad of the bolt catch, the flat surface of the catch that holds the bolt is pivoted back down into its resting position and the bolt moves forward. Now that we understand how things work we can perform some quick diagnostics on magazines. With

An ultrasonic cleaner is helpful in cleaning bolt carrier assemblies, Bob Sled magazines, and dirty chamber brushes. Above center: Use a rust inhibitor in the bore, chamber, muzzle crown, and magazine bodies. the rifle cleared and the bolt forward, "shotgun" the rifle by pushing the rear take down pin out and pivot the front pin with the muzzle down. Insert one of the customer’s magazines and see it lift the bolt catch about 1/4" and pivot back when the magazine is removed.

Any magazine failing this needs to be set aside. Be particularly watchful of any magazines that the owner has identified as not inducing lock back. Those magazines may be pivoting the bolt catch but perhaps not as far up as they should. Reassemble the rifle and further test by letting the bolt forward and locking the magazine in place.

Pull the bolt to the rear with the charging handle to check that the Below center: Conventional 20 round magazine on the left, AR-15 and AR-10 Bob Sled magazines, and a Compass Lake Engineering .22 LR Bob Sled on the right. A critical note. Any time magazines and/or magazine parts are changed there can be point of impact change.

I cover this in detail in "Magazine Checks" (December 2015.) As an example, I’ve experienced a 6 3/4" point of impact change at 300 yards out to the eight ring of the NRA SR-3 the target just by changing magazines. magazine locks the bolt back, setting aside those that don’t. Repair any failed magazines by disassembling, thoroughly cleaning, and inspecting. Pay attention to the magazine interior and follower, noting any burs that tend to snag on your shop rag.

Look for corrosion or dents in the magazine body that may inhibit the free upward travel of the follower. George Harris wrote a particularly good article ("The Art And Science Of Magazine Maintenance") in the March 2014 issue. Apply a very light coating of corrosion preventative such as Boeshield before reassembly, then retest each magazine. Those that still fail need parts (or complete) replacement. Magazine cleaning can be done by shooters, especially after a tutorial by the gunsmith.

The frequency of cleaning can vary from annually to monthly depending upon environment, frequency, and duration of shooting. Any magazine subjected to extremely harsh circumstances like being dropped in the sand or mud should be set aside until it can be cleaned. Even magazines that have passed the quick shop tests that I described aren’t certified as functional until they pass live fire. The shop tests are mostly diagnostics to identify magazines that either need cleaning or replacement.

Start by replacing the spring and follower as needed. Glen Zediker’s The Competitive AR15 Builders Guide has an entire chapter on magazines.

With the bolt pulled fully forward, set the carrier assembly as shown. The carrier on the right has collapsed under its own weight, showing the need to replace the rings soon. Bob Sled magazines are designed for single fed slow fire and even the .22 LR version locks the bolt to the rear for each shot. These exert no pressures on the bolt carrier, promoting more consistency in the positioning of the carrier for each shot.

However, Bob Sleds tend to be a little finicky regarding lock back because most oft hem use a little spring loaded plunger at the rear instead of that comparatively large shelf of a standard mag, making it easy for dirt to get in and compromise movement. I clean my Bob Sleds frequently in an ultrasonic cleaner with the plunger down in the cleaning basket. If the rifle is losing gas pressure there may be insufficient force to compress the buffer spring and allow the bolt to travel that extra 3/8" to the rear.

Check for a loose bolt carrier key. If the key becomes loose, gas will escape and the rifle will run sluggishly. I had this happen in a match in England one time. I had purchased a bolt carrier that came from the factory without the key screws staked and they backed out a little. Standard rings come in sets of three and are split, much like rings on auto cylinders. As with cars, those gaps need to be staggered about 120 degrees from each other. If the gaps line up, gas can escape.

Rings also wear out and need replacement occasionally. To lessen ring wear and enhance accuracy and function, add a few drops of oil to the rings during range firing. Top: Another fix for failures to lock back is to remove the weights from inside the stock buffer. Far Undersized gas ports can also cause issues. Gas ports on issue rifles are 0.093". Many custom gunsmiths and barrel makers start with a drill slightly small and then ream the hole the rest of the way to avoid burrs in the bore.

I have encountered dozens of AR barrels with undersized gas ports that caused sluggish cycling. If the customer says he has had failures to lock back since the gun was new and with multiple mags there is a possibility that the gas port is under sized. Unfortunately, diagnosing this requires removing the front sight. Depending on how the front sight is held in place the shooter may lose his zero. Fortunately, there are easier ways to get the gun functioning.

Similarly, the front sight assembly may not be lined up with the gas port. Again, this is not as uncommon as one might think but, like undersized gas ports, it can be compensated for quickly with the correct buffer spring. I use a simple pointer gauge to check if the sight is grossly out of alignment. We also need to consider the shooter as they certainly can induce malfunctions. With pistols, an infirm grip can induce problems as the slide needs a relatively stable platform on which to cycle.

A similar condition can happen with rifle shooters. In AR-15s the bolt carrier needs a stable platform and that platform includes the shooter and is position specific. If the customer tells

you that lock backs are perfect from prone but experiences problems with the same ammo and magazines from standing, it’s likely the shooter is the culprit. If the rifleman’s position is correct the bulk of his or her weight is directly behind the gun, resulting in a solid platform. Contrast that to the standing position where the center of gravity is high and only the shoulder is directly behind the rifle. Compare muzzle climb in prone compared to standing to see the difference.

We would have identified this problem more easily were it not for the fact that a great number of our rifle shooters get their initial marksmanship training in the military were most of the qualification is done from the prone position while ignoring the others. Having served my share of time in combat I can tell you that terrain, tactics, time, and circumstances don’t always allow us to get into that prone position and our military does a grave injustice by not training more frequently in other shooting positions.

Some additional points to check. The correct buffer spring can cure a bunch of function issues. For example, if your diagnostics find that the customer purchased or loaded ammo that is a little too weak to properly function his rifle it can be fixed by substituting a reduced-power buffer spring. Likewise, if you suspect the firearm isn’t getting enough gas due to an undersized gas port or misplaced front sight housing, the reduced spring will compensate.

It will also usually work if you suspect the shooter’s position is not solid enough for proper functioning of the rifle. For more severe cases, either lighten the stock buffer by removing the internal weights or by substitution of the low mass JP Enterprises buffer. Note, any of these quick fixes can cause shifts in zero. Far The A2 uses a spacer (top right) with a longer screw to attach the stock. Buttstock screws. From the A1 to A2 configuration, the buttstock was lengthened.

The buffer tube, however, was not and a spacer was provided to fit between the back of the tube and the stock. Obviously, a longer buttstock screw was needed. Depending on the source, one can sometimes end up with a screw that is too long and actually protrudes into the buffer tube. If it protrudes far enough the buffer will impact it and not travel far enough to the rear.

If your customer says there has always been a lock back problem with multiple magazines and your hand function check is failing to achieve lock back, you may have a stock screw that is too long. Back the screw out a few turns and if hand function now works, replace with a proper length screw. Shooters with short arms still prefer the older A1 stocks but few factories offer these as standard. A rifle so fitted was likely installed by the owner.

If the aftermarket stock didn’t provide a shorter screw, odds are the owner reused the too-long screw. Recoil reducers come and go from the market frequently. Many of them are similar to automobile shock absorbers and used to counter the secondary recoil pulse portion of total felt recoil caused when the rear of the buffer crashes to a stop at the back of the buffer tube. I have tested a number of these devices and find they work as my recovery times were shortened.

Unfortunately, they can contribute to failures to lock back. In some cases, compensate for this by getting the buffer spring just right. It can be a lot of trial and error. Bolt carrier weights are used to increase dwell time, or how long the bolt remains locked after cartridge ignition. This longer dwell time is thought to improve primer retention

If so, the top of the Wedge can interfere with the bolt carrier.. Above center: Trim the top 1/8" off the Accu- Wedge if it’s in the path of the carrier. This cheek piece was assembled backwards. Remove and rotate it 180 degrees with the Allen screws to the rear. when shooting very hot loads. In .223 Remington you can get an increase of about 0.3 grains of powder by using these. They also influence zero and can negatively impact functioning.

These weights fit into the back of the bolt carrier and add beyond the normal length of the carrier. The manufacturer includes instructions to dress the length of the very back of the buffer to compensate but many owners just skip that step and end up with function problems. Accu-Wedges work. I routinely see guys laying out 100 times the price of that little piece of bubble gum to end up with less accuracy gain! On rare occasions these can contribute to function issues.

Accu-Wedges come in one size and fit most AR-15 lowers but there are variances in lowers and sometimes the "shelf" where the Wedge sits below the take down pin is up higher than normal. This can end up positioning the top of the Wedge’s "tail" section into the carrier’s path. To avoid this, trim about 1/8" off of the top portion of the Accu-Wedge and enjoy the accuracy enhancements with no fear of malfunctions. A dented buffer tube is rare.

Unless the customer has abused the firearm such dents would likely have been in the tube when the gun was originally built. It is not impossible for the tube to have been damaged in shipment and for a careless builder to use it. Don’t rule this out but it doesn’t happen often. Something I have encountered happens with adjustable target stocks for match rifles that clamp directly onto the buffer tube. It is usually the adjustable cheek piece part that gives problems, especially if assembled backwards.

These are held in place by Allen screws that press in on the buffer tube. If a gorilla gun owner torques these set screws down too tightly it is remarkably easy to partly collapse the inside of the thin buffer tube just enough to catch the front end of the buffer itself. I have seen this cause complete stoppages with the bolt carrier prevented from moving far enough. Fortunately, when the gunsmith backs out the set screws the inside of the buffer tube springs back somewhat and function is restored.

Remember, it is the actual front of the buffer itself that becomes trapped, not the rear of the carrier. The carrier was designed so the tail end of it is skinnier than the front. This was done to aid function because the tail of the carrier actually enters the front section of the buffer tube during the cycle of function. The front of the buffer, however, is much fatter than the tail of the carrier.

There isn’t much clearance between the buffer tube’s interior walls and the front of the buffer so it doesn’t take much constriction of the buffer tube walls to create a stoppage. Finally, look for a gross misalignment between the upper and lower. Full rifles purchased from reputable makers are nearly free from this concern. When dealing with "parts guns" or when swapping uppers from different manufacturers I have seen some pretty dramatic misalignments.

If it gets bad enough the bolt carrier can end up scraping the sides of the buffer tube, slowing the cycle of function. A loose buffer tube can also exacerbate the problem. To test for it, remove the bolt, cam pin, firing pin, and its retaining pin from the carrier. Remove the buffer spring from the stock and the hammer from the lower. With the stripped carrier in the upper, reassemble the gun. Hold the rifle in both hands and tilt it so the stock is lower than the muzzle.

The carrier and buffer should freely move to the rear of their own weight without catching or hanging up. Lower the muzzle so it is below the height of the stock and the carrier and buffer should move effortlessly back into battery. While there are a fair number of things that can cause failures to lock back I have found that the actual frequency of this problem is relatively low.

Stoner did a fantastic job of design but if you work on these rifles long enough you will likely encounter a failure to lock back eventually. AG