AR-15 barrel treatments for maximum precision.
I've mentioned many times how disappointed I was in the accuracy of the National Match Service Rifle uppers I inherited when I came on as the Armorer for the Army Reserve Service Rifle Team. Only 39 of 135 uppers would shoot MOA. My purchase budget was very limited so I had to use plenty of elbow grease and ingenuity in finding ways to shrink group sizes. And I wanted to get the guns shooting better with a minimum of delay. While this was a challenge, cryogenic barrel treatment made its contribution.
None of the barrel extensions were properly stabilized inside the upper receivers so the backs of the barrels moved around from one shot to the next. This was common in guns of the era. In order to fix the problem the guns had to be stripped down and rebuilt from scratch, procedures I covered in "The Relationship Of Barrel Extension Diameter To Accuracy In The AR-15" (March and April 2013.) There were other problems like gas tube to float tube contact as well as handguard end caps rubbing against the backs of the front sight housings. These issues were going to require detailed disassembly and adjustments and I knew many of the barrels were coming off their receivers. None of the barrels in question had been fluted.
My budget allowed eight barrels cryo treated for the purchase price of one new premium barrel. Very few of the barrels were shot out and I hoped that cryo would be an affordable repair option. I matched and labeled the barrels to their receivers as I took them off, making sure the guns were put back together with their original parts and retested with the same ammo lots to confirm what the cryo treatment accomplished.
When the gun powder settled, I found that some barrels did not respond at all. I did not cull those barrels out of the final analysis and my total sample of cryo treated barrels improved by an average of 9.4%. I left the non-responding barrels in the final average because there is no way of predicting if a barrel will improve by cryo. If it does, your money was well spent but it's a roll of the dice.
Testing the original barrels found a few "walkers" that fired the first shot from a cold barrel into one place with subsequent shots forming a ragged "straight" line of impacts until the barrel warms up. Such a warm up can require quite a varying number of shots in different barrels. It is not unusual for these shots to "walk" in a diagonal or other direction. In most cases, even when walkers finally do settle down and place bullets into one general area, they are seldom really tight groupers. Walkers seemed to be more common in stainless steel and also most frequent in barrels that had been button rifled. Every time you take a barrel with walker tendencies to the range you have to begin the warm up process all over. Cryo seemed to usually help these barrels.
The "registration point" is the position that the gun muzzle returns to after firing a shot. A good, repeatable registration point is key to tight grouping and a consistent zero. An AR barrel by itself may have a repeatable registration point but if that barrel is simply clapped into the upper receiver in the old 1960s method with a barrel nut, the barrel extension will float around inside the receiver and the hastily-assembled firearm can often give the appearance of a barrel with a registration point problem. Cyro treatment will not help until the back of the barrel is properly stabilized in the upper receiver.
Walkers are likely barrels with a specific registration point problem. They may be barrels that left their factories with some residual stress that hadn't been completely removed. Some barrel makers and gunsmiths have also posited that improper break in of the barrel can contribute to walkers and other registration point problems. The theory goes that barrels heated up by firing too many rounds in rapid succession early in life can result in registration point issues. I discuss this further in, "Gas Gun Barrel Break-In" (October of 2014.)
Since I had experienced some initial success in fixing sick AR barrels, I was motivated to experiment further. I know of shooters and a few gunsmiths who immediately send barrels off for cryogenic treatment in the hopes of harvesting some additional accuracy. I decided to try that. I began with a fairly large sample of dozens of premium barrels that I had just received from several well-regarded custom barrel makers. The sample included chrome moly and stainless barrels with cut and button rifling. All of the barrels had Wylde chambers and I broke them in noninvasively. I sent half of the barrels from each category out for cryo and built temporary uppers with the non-treated barrels. Those were machine rest tested using several known lots of factory match ammo and the results were recorded. The barrels were then cleaned, ranked according to accuracy, and put into storage for later use. When the cryo-treated barrels came back they were also built into temporary uppers using the same receivers, match bolt carriers, float tubes, etc., then machine rest tested with the identical lots of ammo. Testing revealed no difference! The conclusion I drew is cyro has some value as a "repair" option but not an across-the-board improvement.
I see in some catalogs where certain gun makers tout up that their barrels are cryo treated. Often this is accompanied with grandiose "sub minute" accuracy claims that I have never seen delivered. Don't be surprised if other promises like "triple lapped" accompany their write ups. I am reminded of the 239th Ferengi Rule of Acquisition, "Never be afraid to mislabel a product." Perhaps cryo is an informed decision on their part. Maybe they are buying barrels from low-bid makers not doing a proper stress relief at the factory. Such barrels may benefit from being cryogenically treated. Or perhaps such companies have just failed to perform the due diligence that I did. The cost of having low-bid barrels treated with cryo is being passed on to their customers. Since not everyone is informed on the true values of cryo they may expect to get superior performance just because of it. That was certainly not my experience given an average 9.4% group reduction. For me, cryo merely brought marginal barrels up to shooting average. If you are looking for barrels with true match-grade accuracy I recommend buying a genuine premium barrel from reputable companies like Krieger and Bartlein and not gamble on cryo, fire lapping, or other work arounds.
There's a tie in with fluting and barrel cryogenics. Fluting looks cool (not a valid reason to get it) and cuts barrel weight while maintaining rigidity. While most Service Rifle competitors usually want to add weight, tactical shooters or hunters may find the weight reduction attractive. My interest had to do with claims of improved heat dissipation much like a car radiator with added surface area. National Match competitions are fired on round bullseye targets at distances of 200, 300, and 600 yards with long range at 1,000 yards. The National Trophy Infantry Team Match ("Rattle Battle") is fired on human silhouette targets and starts at 600 yards. Staring from sling-supported prone, shooters have 50 seconds to fire as many rounds as they can. Better shooters will expend an entire 30 round magazine. From there, the team advances to the 500 yard line and does it again. Should the team have remaining ammunition they'll repeat at 300 yards. Obviously, these barrels get really hot and it's not unusual for hot barrels to shift point of impact and shoot fatter groups. Heat mirage from hot barrels can also trick a shooter's eyes. I wondered if fluting might help with barrel heating.
As with other experiments, I started with a large sample of new button-rifled barrels from reputable custom manufacturers and sent half out to be fluted while building the others into temporary uppers. I tested from machine rest using known lots of factory match 77 grain ammunition by firing 30 rounds as fast as I could push the machine rest back into battery. Then the target was rapidly swapped out and a second magazine with a different ammo lot was fired, rather like a "rattle battle" match. When the fluted barrels came back I removed the unfluted control barrels and retested. Groups were terrible with the fluted barrels. The folks that did the fluting for me cut a fairly aggressive flute, with half of the barrels done in a straight pattern and the other half in a spiral. The flutes were cut fairly wide and deep, which should have produced the desired increased surface area for cooling.
Why did the fluted barrels shoot so bad? There are two possibilities. First, changing the exterior of a barrel that has been rifled using the button process. Button rifling is a swaging process. The groove is not actually cut out, rather, a button is pulled through the barrel and the groove is displaced into the barrel. The theory is that when the exterior of such a barrel is changed, dimensional changes to the inside of the barrel occur and alter what the barrel maker intended to be final. Some barrel makers recommend that if a customer desires fluting that he allow the manufacturer to do it under their controlled conditions so they can perform a final lap after the fluting to help eliminate internal dimensional changes. This sounds good but not all custom barrel makers lap their barrels. Those that do don't always offer factory fluting.
The second possibility for poor performance has to do with inducing stress. It is possible that the folks that performed the fluting rushed the operation and heated things back up, inducing stress. I am inclined to favor the second possibility. The end result of my fluting experiment was a bunch of sick barrels. I sent them out for cryogenic treatment, then reassembled them back into the same uppers for a second round of testing. Every one of them did better, however, the cryo-treated fluted barrels still didn't outperform the non-fluted control barrels. So I spent a bunch of taxpayer dollars to have barrels fluted and then fixed with cryo to achieve no accuracy gain. While I am sorry to have wasted your money there is always some cost to learning.
I would caution to not draw the wrong conclusion from my specific fluting test. Fluting did not work (heat wise) for my NMC uppers where the barrels are surrounded by a close-fitting metal float tube. These tubes remove sling pressure from the barrel and the first models were much better ventilated on the top and bottom. I suspect that the makers were considering barrel heating in their designs. Over the years, a lot of the vent holes and slots have been dropped. This was likely just an economic consideration on the part of their manufacturers as it takes extra time and machining to drill and cut these vents. Also, shooters have added more lead weights between the tubes and handguards which also holds barrel heat in and not let it dissipate.
Hunters and tactical shooters probably have firearms that better lend themselves to ventilation and fluting might work better for them, especially if they also want to reduce weight. Match Rifle, F-Class, and some long range handguards are also better vented than those for Service Rifle. I don't know if fluting helps in these cases as I have only tested with Service Rifles.
A caveat on the lead weights for Service Rifles. In addition to the bottom handguard weight, some shooters also put an upper lead weight in place. Be very careful with this as these upper handguard weights have a channel cut in the lead to avoid contact with the gas tube. In reality, often there will be contact, bending the tube slightly out of alignment with the bolt carrier key and causing fliers and fat groups! If you are contemplating using an upper weight, smoke the top of the gas tube with a carbide light, place the weight on top, and check for any black carbide soot rubbed off. If that happens, I'd abandon the weight. While the extra weight helps with recoil control, the degradation in accuracy isn't worth the price.
I did have a cut-rifled Badger barrel fluted. Unfortunately, Badger is no longer in business but Northern Competition (NorthernCompetition. com, 262/857-8762) did the fluting for me and the owners used to work for Badger. They also had concerns about inducing stress by rapid and hot machining of the flutes and use a very noninvasive method. They make light cuts and rotate to a different flute constantly to avoid overheating. The 6.5 twist 90 grain barrel done by them shot great without cryo or other work. I can't say if it was their noninvasive fluting method or the cut rifling but fluting did not impact accuracy in this barrel. If you are looking for someone to flute a barrel for you they might be worth contacting.
I see no reason to flute National Match barrels. If you are going to have fluting performed, the best way is to have the actual barrel maker do it at the factory so they can perform a final lap. If you are performing the actual fluting yourself, do so without heating up the barrel. If you have a fluted barrel that is not performing well you may want to consider shipping it off for cryogenic treatment.
Used judiciously, cryo may help with problems like walkers and stress-induced accuracy issues. Companies offering the cryogenic service used to make other claims for it like easier cleaning or longer barrel life. I tested cryo for accuracy, not for ease of cleaning, which isn't easily quantified. Alleged longer barrel life is a real can of worms and after examining hundreds of barrels I am convinced that "wear out" is as much a function of how the owner cleans the bore as the actual round count.
Because of that extra cleaning variable thrown into the mix it is very difficult for any researcher to set up a study of how cryo influences wear out. If barrels did last longer from cryogenic treatment I didn't notice it. I prefer to go with true premium barrel makers such as Krieger and Bartlein that don't induce stress or do a good job of stress relief at the factory and I avoid any machining or questionable break in procedures that might reintroduce stress. Cryo isn't all things to all people but it may be of value to you as long as you have reasonable expectations of it and apply it to those situations where it can really make a difference.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||TECH TOPICS|
|Date:||Sep 1, 2016|
|Previous Article:||Savage 167: gunsmithing the Savage 167 pump action shotgun.|
|Next Article:||Revolver repair: important and common things to check when working on revolvers.|