Assessing Barrel Wear, Part One

How long will anew barrel last? How long will its accurate life be? Glen Zediker in his book, The Competitive AR-15, summed it up best: "No one has an answer for how long a barrel will last. No one." Two of the most common questions I get as an AR-15 armorer are, "How far should I jump my long range bullets?" and "How many rounds is my AR-15 barrel good for before it shoots out?" We will cover the first question in a future article.

I usually answer the second with, "I have seen brand new barrels go out in as little as a few dozen rounds while others last into five figures." The person asking assumes I am either being flip, evasive, or that I just don’t know. I plead "Not Guilty" to the first two but admit freely to the third assumption. As Mr. Zediker said, no one else has the answer either. Most barrels wear out in the throat area where the bullet starts contacting the lands.

Intuitively, most folks think that friction is the culprit, however, most barrel makers feel it is the hot, erosive gasses eating at the metal. Gradually, the point of bullet contact with the lands moves further forward, making bullets jump greater distances without any support before they enter the rifling. These same gasses also cause a gradual roughening of the back of the barrel which can be abrasive to the bullet. Often, a condition called "fire cracking" will occur.

Actual rough cracks form, usually running with the "grain" of the barrel. A bore scope is a handy tool when performing diagnostics on a barrel. Wear, especially fire cracking, can look very ugly in the scope, however, I find that a fair number of unsightly barrels continue to shoot well in spite of appearance. With the bore scope, be on the lookout for any lands wearing at a faster rate than the others back in the throat area. This may allow bullets to tip away from the center axis of the bore.

Such uneven wear may be caused by a less than stellar chambering job or cleaning rod damage. Barrels normally wear out at the throat. In a 20-inch Service Rifle barrel, the forward 18 inches of rifling usually shows very little wear, contrary to the normal perception that barrels wear out due to friction.

I have literally shot out a number of Service Rifle match barrels, cut 5.5" off the back, rechambered, threaded and reinstalled the barrel extension, arriving at a carbine length barrel that shot just as well (with a shorter carbine-length gas tube) and just as long as the original parent barrel! You can perform a similar sleight of hand when a 26" March Rifle barrel shoots out by converting it to a 20" Service Rifle barrel. Why throw away a $400 investment when there is lots of life left in it?

It would be like hauling your car to the junk yard just because it got a flat tire. Throat wear can be quantified either by crude throat gauge readings or in a bit more detail by taking Stoney Point readings. The Stoney Point, now made by Hornady and called an "Overall Length Gauge," is a necessity for VLD bullets which require nightly precision measuring of throat erosion and setting the bullets out longer each day to compensate.

Called "chasing the throat," this can result in stellar performance but requires diligence and attention to detail. Predictors "Aren’t you beginning to feel time gaining on you? It’s like a predator. It’s stalking you. Oh, you can try and outrun it with doctors, medicines, new technologies, but in the end time is going to hunt you down and make the kill." –Dr. Tolian Soran, Star Trek VII: Generations How long before time hunts down your barrel and makes the kill?

I can provide some subjective and general rules of thumb that will help you a little. Predictor 1: Barrels with tall .218" lands will usually last longer than barrels with the more common .219" lands. Predictor 2: Barrels with wider lands will usually wear better than narrow land tubes. Predictor 3: The type of powder will come into play. Usually ball powders burn a little cooler than stick powders.

The stick powders that burn the hottest and damage bores the most severely seem to be the imported, "high energy" types. I tried these powders in button rifled chrome-moly barrels when I first started working up loads for 90 grain bullets and they ate the barrels for lunch. The backs of the barrels were rough and fire cracked and often only lasted a mere 500 rounds before

Starting from the top is a bore scope. On the right, a throat gauge for the AR-15 and its big brother for .308 M1A on the left. Below that, a Stoney Point gauge and digital caliper. accuracy was completely lost. I also found that the high energy powders gave poorer chronograph standard deviations and shot fatter groups than the cooler burning "regular" powders. One company making these high energy powders claims higher velocities with lower pressures. I found no evidence of that.

In fairness, I admit to not having chamber pressure measuring equipment and use primer retention as my judge and grant this is not as accurate as expensive lab grade pressure testing equipment. Predictor 4: About 20 years ago Boots Obermeyer told me that in the .223, with the powders we use and the pressures we operate at, he felt that stainless barrels would erode smoother than chrome-moly. The experience with imported, high energy powders seemed to bear that up.

After I started wearing out chrome-moly button rifled barrels at only 500 rounds I switched to button rifled stainless and my round counts went up two or three fold. With conventional powders, however, I see little to no difference in the roughness of erosion between the two steel types. If for some reason you use the imported high energy powders, I recommend stainless barrels. If you use "regular" powder I see no reason to choose stainless over chrome-moly.

Predictor 5: If your gunsmithing business ever gets in a slump and you are hard up for cash, get two barrel makers together in the same room and ask them who uses the better grade of steel and whose barrels will last longer. You will be able to sell all the tickets you have seats for and pocket the cash because I guarantee a real fight is going to break out.

The cut-rifled barrel makers will tell you that the button-rifled camp has to use softer steel because they aren’t removing any metal in making their grooves, they’re displacing metal by swaging. The button-rifling companies will jump up and down and swear that their steel is just as good and that their barrels will outlast the cut-rifled barrels because the swaging process allegedly leaves a greater surface hardness in the bore, which sounds plausible.

I’m not going to try to referee the squabble between barrel makers but I will refer back to my experiences with high energy powders. When the chrome-moly button-rifled barrels started to fail at those low 500 round count figures, in addition to switching to stainless button-rifled barrels I also tried cut-rifled barrels of both steels. I had no problems with the cut-rifled barrels in either steel.

In general, I tend to agree with Glen Zediker once again in that I believe cut-rifled barrels last somewhat longer than buttonrifled ones but am unable to quantify how much. Predictor 6: Barrels with chambers born with shorter throats will significantly outlast those with longer throats. You will likely see this more with magazine-length ammo than with long range ammo that is fed one round at a time by hand.

Since the long range ammo doesn’t have to fit in a magazine the bullets can and should be seated much further out, nearly contacting the throat. The shorter jump contributes significantly to accuracy. Predictor 7: Slow twist barrels will last longer than fast twist ones. Having bullets "stall" at the back of the barrel increases both pressure and the time that those hot erosive gasses

are eating away at the throat. Bullets "stall" because of inertia. The resting bullet does not want to move forward nor does it want to start rotating at a couple of hundred thousand RPMs. Slow twist barrels offer less resistance to the rotational portion of inertia and the bullet moves out of the throat quicker and with less pressure. If you shoot long bullets you have to spin them faster to get them to stabilize. Eighty grain bullets want a twist rate of about one revolution every eight inches.

The longer 90 grain bullet needs a faster twist rate of 1:6.5". To get those long, heavy bullets moving out of the throat faster you may want again twist barrel. I am using gain twist barrels in all 90 grain guns I build and my customers and I like them a lot. In 20" Service Rifle length barrels I begin my twist at the front of the chamber at 1:13" and finish up at the muzzle at 1:6.5". In 26" match rifle barrels I start at 1:14" at the back and terminate at 1:6.9" at the muzzle.

For more information on all the advantages of gain twist barrels, refer to my article, "Revisiting Gain Twist," in the November and December 2013 issues. If you want to shoot 80 grain bullets the above barrels will work fine or you can use again twist starting at 1:16 in front of the chamber of a 20" Service Rifle length tube and terminating at 1:8 or 1:7.7" at the muzzle.

To see again twist barrel that has been split in half lengthwise, visit my store front on Commercial Row (Bldg. 910-A) at Camp Perry during the Civilian Marksmanship Program Service Rifle matches. Predictor 8: Barrels that are cleaned properly and frequently usually last longer. If you clean your barrel following every range session you will get the fouling out easier and before it builds up. Neglected barrels will have heavier build ups of fouling, often requiring the use of abrasive cleaners to get clean.

I don’t worry about the abrasives themselves but they are usually used in a back and forth stroke where the direction of the patch is reversed inside the bore. This can result in some awful cleaning rod flex, causing the rod to scrape the lands and wear at them. Those lands are only about the thickness of a sheet of paper above the grooves and can be easily damaged!

Predictor 9: Shooters who spin their cartridges for concentricity and who manage run out will see longer service from their barrels than those competitors who just take the ammo out of the box at random. The popular Wylde chamber jumps 77 grain, magazine-length ammo about 0.116" when it is new. That is a lot! Imagine that same gun a year or two down the road when it has a couple of thousand rounds on it. Allowing for throat erosion the jump could easily be 0.25" by then.

Perfectly concentric ammo has the bullet lined right up with the center line of the bore. Ammo with run out (bullets not seated perfectly straight) will send the bullet off at an angle to the center of the bore. Picture it like a water hose. A bullet fired from a car-

tridge with run out in anew Wylde chamber is going to enter the bore off center and be deformed. If fired in a worn Wylde chamber that now has 0.25" of jump is going to hit further from the center of the bore and it is going to be traveling faster. The combination will booger it more and it will hit further from where you intended it to. Look at the two test groups pictured here. Both were fired from the same gun on the same day using the same ammo lot.

The barrel in question was worn to a bullet jump of about 0.25". The fat 1-7/16" group at the top was fired with 77 grain, magazinelength ammo selected at random from a box of 50 rounds. The 3/8" ten shot group at the bottom was fired from the most concentric 10 rounds of the 40 remaining cartridges in the box. No barrel maker can produce a barrel that will shoot optimally with crooked ammo and no gunsmith can make a gun that will perform well with bad ammo either. Concentricity management is not new.

To learn more about the subject visit the Nielson Brothers web site (NielsonBrothersArms.com). Their research was done with .22s but their conclusions and their predictors are equally valid to center fire rifle and handguns. Smart hombres! In reviewing their research pay particular attention to the text and pictures regarding how the orientation of the base of the bullet contributes to accuracy.

It is fairly intuitive and easy to mentally picture the tip of a bullet seated off center in the casing crashing at an angle into the back of the barrel, causing accuracy damage. We are less inclined to think about the base of a bullet seated at an angle in the casing, however. The bullet base needs to be perfectly perpendicular to the axis of the bore, especially when it exits the muzzle.

Depending on the degree of run out, the base of a bullet seated crooked in the casing will be out of plum with the bore to just as great a degree as the tip, we just don’t think about it. When the base of a bullet that is not perpendicular to the axis of the bore exits the muzzle, part of the base will still be inside the muzzle while part of it has exited due to being crooked. High pressure gasses will also exit at the same time on that side and will push the bullet at an angle.

That push will not be in line with the center of the target and will cause a flier or just contribute to a fat group. This is a condensed version of what is on the web site and their pictures really help grasp the contribution that bullet bases and concentricity make to group size. Round Counts Some of the military teams use round counts to determine when to rebarrel a gun. When I was the armorer for the Army Reserve I occasionally got questions about the applicability of that scheme to the Reserve environment.

I doubt that it would work for a reserve team with shallow fiscal pockets and I discourage civilian shooters as well. Arbitrary round count barrel rotations have to be set low in order to catch the guns that shoot out early. For example, if I test ten given barrels and one of them shoots out at 1,500 rounds while the other nine go all the way to 4,000, I would have to set my rebarreling schedule at something slightly below 1,500 to make sure I catch all of the worst case scenario barrels.

In this example I would be removing nine perfectly good barrels capable of shooting match winning scores for another 2,700 rounds each and wasting about $4,000. How far from reality is this? Years ago I bought about a dozen stainless barrel blanks from a well known barrel maker. All of them were button rifled the same day in one special run and final machining was done at the same time on the same equipment, with my Wylde reamer used to chamber all of them.

When they arrived at my shop I broke them all in using non-invasive techniques. I then I installed the barrels, worked up loads, and issued them to two different shooters, both using 90 grain bullets for long range. Shooter A loaded with a high energy powder, cleaned his barrel with one of the new, harsh "gee-wiz" solvents and the barrels were completely shot out after 500 rounds. Shooter Bburned cooler Varget powder, used Hoppe’s solvents and the barrels were still going strong after 1,500 rounds.

In fact, there was no accuracy fall off as when new! Why did one barrel shoot out so prematurely while the other gave more normal wear and performance? Was it the hot burning powder or solvent? Or was it some unrelated variable that we failed to identify? It is likely that none of us will ever know but the whole scenario causes me to have little faith in trying to predict barrel wear out based on round counts.

If you are still not convinced and hear that big military shooting teams (with deep pockets) is using a specific round count for rebarreling rotation, ask the following. What is their chamber length? Let’s say they are using along 2.50" chamber and you are using the shorter (and likely better) Compass Lake one at 2.45". All things being equal, their barrels are going to shoot out earlier than yours and if you follow their rotation scheme you will be rebarreling much too early.

What brand of barrel are they using and which rifling method does the manufacturer use? What is the land to groove ratio of their barrel compared to yours? What is the land height? How often do they clean and what solvents and procedures do they use? Do they manage the concentricity of their ammo and do you? Do they chase the throat and seat long range bullets out further to compensate for erosion? Do you? What kind of powder do they use? What is the twist rate of their barrels and is it conventional or gain twist?

Given these variables, do you still want to follow someone else’s round count rotation for rebarreling? We’ll discuss more indicators to help assess and prevent barrel wear in Part Two. AG