AR-15 Upper Quick Fixes: Match Bolt Carriers

When I became the armorer for the Army Reserve Marksmanship Program Service Rifle Team I followed the advice taught in various military leadership schools. I assessed what we had, machine rest testing all the uppers I inherited. Average group size was over a minute of angle and only 39 out of 135 rifles would shoot one MOA or better. As in Shelley’s poem, I despaired! I expected much better. In connection with this initial assessment I recorded all group sizes and throat gauge readings.

While most of the barrels were not shot out, the uppers just weren’t shooting well. From having been in the firearms business for decades I knew it could take up to a year or longer to get enough replacement barrels. Seeking a quicker and cheaper fix, I eventually cut average group sizes in half but it involved a lot of work coupled with a fair amount of trial and error. Match bolt carriers contributed their share of the improvement. Visual inspection of the Army Reserve uppers revealed glaring errors.

Lots of gas tube and float tube contact, front sight housings contacting the front of the float tube hand guards, broken float tubes, among other things. With the exception of the latter, most of those faults could be corrected cheaply with hard work and long hours. I began by tearing most of the below average uppers down to their component parts and carefully reassembling them.

See my two part article in the March and April 2013 issues, "The Relationship Of Barrel Extension Diameter To Accuracy In The AR-15" for details on how to stabilize the barrel extension in the upper receiver. This alone resulted in an 18.6% average reduction in group size. Now that I am using the great BAT Machines (batmachine. com) oversized match barrel extensions I am realizing closer to a 35% reduction in group size. I read up on match bolt carriers. There seemed to be two major claims made for them.

First was a slick finish. This would amount to better polishing of contact surfaces and/or a finish different from standard bluing having a lower coefficient of friction. The finish could be hard chrome or some gee-wiz self lubricating finish. It was claimed that the low coefficient of friction would result in more consistent bolt carrier velocity and more consistent lock ups.

Having accurized M9 and 1911 pistols in a previous life, I had already documented the importance of consistent slide velocity in enhancing accuracy. It made sense that consistent bolt carrier velocity should result in tighter rifle groups as well. The other attribute of match bolt carriers was for either increased bearing surface and/or fatter dimensions that, again, were purported to result in more consistency in lock ups and positioning.

I had observed high end bolt gunners and their gunsmiths paying a lot of attention to bolt lock ups in their gun building. I figured they had good reasons for their diligence and that applying the same principles to gas guns could harvest some accuracy. I don’t consider a carrier to be match grade unless it has a slick finish and greater or fatter bearing surface.

Just dipping a regular carrier in some hard chrome solution or making it out of stainless without incorporating bigger dimensions does not qualify it to true match carrier status. Obviously, some manufacturers have their own ideas and marketing strategies as there are "match" carriers advertised that don’t fit my criteria. Since I can’t issue Executive Orders they are free to do as they see fit. I use three carriers in my building meeting my criteria: JP Enterprises, Young, and Les Baer.

As the Army Reserve Team armorer I had a modest budget for gun parts. In the early 2000s I could purchase about four match bolt carriers for the cost of one premium match barrel and, unlike good barrels, there wasn’t along wait for the carriers. I purchased a few dozen match carriers with keys installed and went to work. The process wasn’t esoteric. I just machine rest tested each gun with several known lots of ammo and the stock carrier. Then I substituted the match carriers and retested using the same ammo lots.

Which ever carrier shot the tightest aggregate group size was the one left in the gun. Most of the uppers responded favorably to the match carriers but not all of them. Asking why some match carriers failed to improve groups or sometimes made the gun shoot fatter groups than the standard carriers, I

From front to rear, Young, JP Enterprises and Les Baer. believe the carrier influences barrel harmonics. This makes sense in as much as the carrier is locked up via the bolt with the back of the barrel as the bullet is traveling forward and exiting the muzzle. Match carriers are meatier, up to a full ounce or so heavier than a stock carrier, and I believe this changes barrel harmonics. The same thing happens with bolt carrier weights inserted in the back of the carrier are installed.

Sometimes the gun shoots better but, about as often, groups get fatter. The purpose of the weights is not to improve the accuracy of the gun, they are used to increase dwell time—how long the bolt stays locked up—which can help with primer retention when shooting hot loads. I see shifts in point of impact when substituting different carriers or inserting bolt carrier weights, supporting my idea this influences barrel harmonics.

If you are installing either device in a customer’s gun, stress the need to recheck zeroes. Caveats Some guns performed worse with match carriers than with the original stock part and I feel this is due to barrel harmonics. Those not seeing accuracy improvement likely had more consistent bolt lock ups but that was canceled out due to bad harmonics.

You can influence Stoner platform rifles by changing pressures and angles applied to the barrel or by making ammo changes, varying powder charges or bullet seating depth to hit a "sweet spot." Just because a match carrier doesn’t immediately improve a gun doesn’t warrant giving up. Function, cleanliness and lube. A couple decades ago as a firing member of the Army Reserve Marksmanship Program, before I became a professional gun plumber, a conversation on the range at Camp Robinson.

This back when the Army Marksmanship Unit Custom Firearms Shop at Fort Benning was beginning to experiment with the AR-15 for National Match competition. It was so early in their development project many members of their Team were still shooting the old, high maintenance M14s. Mr. W. O. Harden, then chief of the AMU CFS, was present and members of other service teams were picking his brain about their progress.

Someone asked him how often the AMU was cleaning their guns and he answered, "Every time we shoot them and that includes the bolt carrier assembly." This was before match bolt carriers had been invented, with everybody using stock carriers. Without thinking, I blurted out that I believed my AR- 15s would function flawlessly if the bolt carrier assemblies were only cleaned every 1,000 rounds. I was only a Staff Sergeant at the time, saw Mr.

Harden take a minute to size me up and figured I was going to get a good butt chewing. Much to my surprise, and very much to his credit, he addressed me by name and rank and, with more patience than I deserved, explained the reason they cleaned their bolt carriers daily had less to do with function and more to do with accuracy. The CFS gunsmiths believed keeping the carrier assembly clean and well lubed gave more consistency in lock ups and pressures on the cartridge and orientation of rounds in the chambers.

I began to pay more attention to the cleanliness of my bolt carrier assemblies and I included nightly cleaning of my chamber and barrel extension using the appropriate chamber brush. By doing so and relubricating frequently I noticed my shoulder fired groups were

tightening up. When testing and tuning Stoner platform guns on the machine rest, I begin each session with an immaculately clean bore, chamber and bolt carrier assembly, adding oil through the holes in the carrier every 20 rounds. Was Mr. Harden overstating the importance of weapons cleanliness all those years ago?

If you ever get the chance to visit Camp Perry during the National Rifle Matches, walk by the military trailers at the end of the shooting day you will still see the AMU shooters diligently cleaning their rifles. Then walk past the scoreboards and see which team is normally on the top or very near.

Weapons cleaning alone won’t make Young (center) does not stake, while JP Enterprises (right) stakes each side. 078") between the top of the carrier and the receiver indicating bolt carrier tilt, a situation that robs accuracy. you a national champion, nor will it turn a sow’s ear of a gun into a silk purse. The AMU wins often because they have talented shooters and they train often. They also dot their equipment i’s and cross their cleaning t’s, thus leaving little to chance.

Fast forward to the early 2000s when I was working on the Army Reserve Service Rifle Team ARs. Knowing match bolt carrier manufacturers made dimensional changes to Eugene Stoner’s design, I was hesitant at first in substituting them for fear of inducing malfunctions. I was pleasantly surprised when my team members reported no problems with the improved carriers. In fact, they liked the ease with which the polished surfaces cleaned up. That said, however, they are fatter and have more bearing surface.

I can’t overemphasize the need to keep them clean and well lubed. I recommend nightly cleaning of the carrier assembly. An ultrasonic unit makes this less painful. Keep

the area where the carrier lives in the upper receiver followed up with plenty of lube. Just be careful not to over lube the parts of the carrier near the ejection port so you don’t get excess spraying out. On the range I like to see a little lube going on the rings of the bolt every 20 to 40 rounds.

Pull the carrier back a short distance with the charging handle until you see the rings line up with the two vertically spaced holes in the side of the carrier and put a couple of drops of oil through each hole directly on the rings. It is best to position the upper on the left (bolt carrier release) side when doing so. Wipe off any excess that gets on the side of the carrier. This is not just a function issue but an accuracy influence as well so we can get those consistent bolt carrier velocities and lock ups.

How much did the substitution of the match carriers help? My records show an average 8.8% improvement, which is on the modest end but needs a little explanation. I arrived at that figure not only taking into account the majority of uppers that shot better, but the ones that degraded as well. I wanted to see what a person without knowledge of the influence harmonics in the AR-15 could expect by just a simple substitution of the better carriers.

My attitude as the armorer for a big military shooting team was to take all positive gains I could get and add them with other gains. This eventually led to cutting group sizes in half. If you are building a gun from scratch and need a carrier anyway, why not buy one that helps shoot tighter groups? The difference in cost between match carriers and garden variety ones is not that great.

Carrier Key Staking I covered this in detail in a previous two part article on troubleshooting ("Do You Have Gas?", January and February 2014) so I am only going to skim over it now. JP Enterprises and Les Baer both stake their key screws on to their carriers. The military also stakes theirs. Young elects not to but uses an epoxy and slightly greater torque on their screws. I suspect that many of your customers will expect and insist on staking.

I had an unstaked key (not a Young) come loose during a match in England and it gave me fits. If you buy a Young carrier, realize that staking will void their warranty and they include a detailed write up with the carrier stating their reasoning. If you elect to stake, a tool from Brownells will help get it right. Bolt Carrier Tilt Look at the back end of an AR-15 where the rear of the carrier sits. Insert your finger in the back of the carrier and you will quickly see that it can be moved in all directions.

Obviously, as you are doing so you are also moving the front of the carrier which holds the bolt, moving the bolt face as well. This has been a major detractor in AR-15 accuracy from the get go. Match carriers attempt to compensate for this by being fatter at the front. Certainly they help but all of them ignore any improvements to the back of the carrier where it is so desperately needed. So we aren’t finished with the very interesting topic of improving the carrier by any means.

This is a good jumping off point in the discussion, however, as the topic of options to combat carrier tilt is a full article by itself. AG