Factors in Barrel Life: Throat Erosion Part I

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The preceding was a specially adapted excerpt from the book The Competitive AR15: the ultimate technical guide, by Glen Zediker and Zediker Publishing. Available at Midsouth Shooters or BuyZedikerBooks.com

By Glen Zediker

Last time the topic was finding the bullet seating depth that touches the lands or rifling. In discussing the tool featured, the Hornady LNL O.A.L. gage, I also mentioned how this appliance can also be used to document barrel throat erosion. A little more about this…

As rounds go through the barrel, one after another, the chamber throat is advancing, moving toward the muzzle. The “wear” in a barrel is all right in the throat. The influence is “flame cutting” by the high-temperature gases that result from burning propellant. The steel is burning up. At some point, it quits shooting well. The reason for a fall-off of accuracy is a combination of excessive free-bore (space between end of the chamber neck area and the first point of contact a bullet will have in the barrel) and also roughness, plain and simple. Bullets won’t enter the rifling as smoothly and the rough surface rips at the bullet jacket. Any wear along the remaining length of the barrel is insignificant, and not influential; it’s mostly from simple friction.

So how long does a barrel last? About 5 seconds. Let me explain.

Of course, that’s spread over a scant few milliseconds at a time over a number of rounds. There are two main influences in the progress of erosion: bullet weight and amount of propellant. The more of each, the faster the deterioration, but bullet weight factors mostly. As was introduced in the material way on back about gas port pressure, if we plot out pressure levels against bullet movement through the bore, we get a “pressure-time curve.” Pressure levels are associated with respective levels of flame cutting. A steep P-T curve (slow bullet acceleration) means more concentrated cutting over a shorter distance. It’s clear, then, that lighter bullets will do less damage than heavier bullets, even though the lighter bullets mean burning more propellant. In a .223 Rem., for example, a steady diet of 77-grain bullets will shorten barrel life compared to using mostly 55-grain bullets. Fortunately, .223 is one of the kindest to barrel steel of popular rounds. I expect about 5000 good rounds from a quality barrel (about the same as .308 Winchester). Some folks offer a much higher figure than that, but, again, “shoots well” is subjective. Shoot it until it doesn’t shoot well. The cartridge factors mightily: .243 Win.? Maybe 1200 rounds with 107-grain bullets.

Barrel steel material most definitely has an influence on life. Short answer: get stainless steel. Comparing true “match-grade” barrels, stainless will not shoot one bit better than chromemoly, but will shoot its best for longer, about 10-15 percent more accurate rounds. The reason is in how the steel “wears” as throat erosion progresses. Chromemoly tends to get rough (like sandpaper) whereas stainless steel tends to form cracks with still-smooth areas between them (like a dry lake bed). The stainless erosion is less disruptive to the bullet jacket.

However, that’s not a blanket recommendation of stainless. When a stainless barrel quits shooting, it quits right then and there. Accuracy fall-off is abrupt. Like in the middle of a string… Really. Chromemoly group sizes cone outward more slowly. Chromemoly tends to continue to shoot “better” after it’s lost its gilt-edge. Some will shoot a very long time at only a minimum fall-off from its best performance. I have to recommend chromemoly for a semi-auto, unless, that is, the semi-auto is strictly a competition rifle. Then, just take your medicine.

A bolt-gun can have its chromemoly barrel pulled and “set back” to prolong its life. Simple: cut a half-inch or so off the chamber-end of the barrel, rechamber it, back in business. Can’t do that with stainless. Of course, semi-automatics can’t get this treatment because of gas system orientation.

Chrome-lined barrels do, yes, tend to last longer (slower erosion), but they also tend not to shoot as well, ever. Steel hardness also factors, but most custom match barrels are made from pretty much the same stuff.

Back, finally, to using the gage: Take a measurement every now and again. Experience will tell you when. My standard is +0.150 inches, compared to the reading I got on the barrel when new. That’s a max. Sometimes they quit before then, sometimes not, but no matter how it’s shooting, I rebarrel at +0.150. I use stainless and I don’t want the next target to be when it quits. I’m a little conservative, by the way… Some very good shooters comfortably extend that to 0.250.

Keep a worn barrel clean! Scrub the throat area (carefully of course). The cracks and roughness attract and retain fouling. There are other “tricks” to help preserve accuracy for a longer time, and I’ll talk about those next time.

 

Use the Hornady LNL O.A.L. gage to record and then track barrel throat wear. This isn’t technically a “throat erosion gage,” which do exist, but the author has found it an easy and reliable way to keep up with an advancing throat. As the seating depth gets longer, it’s indicating how far the throat is advancing.
Use the Hornady LNL O.A.L. gage to record and then track barrel throat wear. This isn’t technically a “throat erosion gage,” which do exist, but the author has found it an easy and reliable way to keep up with an advancing throat. As the seating depth gets longer, it’s indicating how far the throat is advancing.

 

Bullet weight, mostly, determines barrel throat life. Why? The heavier the bullet, the slower it accelerates, and the more time the flame from burning propellant has to torch into the metal. Even though a lighter bullet is burning more propellant, it’s the intensity of the cutting that does the most damage.
Bullet weight, mostly, determines barrel throat life. Why? The heavier the bullet, the slower it accelerates, and the more time the flame from burning propellant has to torch into the metal. Even though a lighter bullet is burning more propellant, it’s the intensity of the cutting that does the most damage.

 

Stainless steel barrels keep their “gilt-edge” accuracy for about 15% more rounds, but hit the wall head-on and in a big way when they reach their limit. Chromemoly steel tends to open up groups sooner, but also maintains “decent” accuracy for a longer time, in the author’s experience — the groups open more slowly. Most are best served with chromemoly in a semi-auto.
Stainless steel barrels keep their “gilt-edge” accuracy for about 15% more rounds, but hit the wall head-on and in a big way when they reach their limit. Chromemoly steel tends to open up groups sooner, but also maintains “decent” accuracy for a longer time, in the author’s experience — the groups open more slowly. Most are best served with chromemoly in a semi-auto.

 

 

 

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18 thoughts on “Factors in Barrel Life: Throat Erosion Part I”

  1. I’m confused as to why you say a stainless barrel can not be “set back”. I had mine set back and it reduced the free-bore and definitely restored some lost accuracy.

  2. Good info; thanks!

    In the article you talk about the relationship of the pressure-time curve to the relative wear of the barrel. Is it the peak pressure that indicates more/less wear, or the “area under the curve”?

    In other words, if you were to use a slower burning powder that developed less peak pressure, but burned for a longer time as the bullet traveled down the barrel, would the “flame cutting” be less than a faster burning powder that developed a higher peak pressure, but for a shorter length of time? Or would the wear be about the same?

    1. You’re mostly correct in your assumption. There can be problems, you should always use loading data to select your powders and bullets. Don’t use any powders or bullet weights that you don’t have reputable load data for.

      Mostly these articles have more information that I or the average shooter will ever use. They should just say that if you’ve put a few thousands rounds through your barrel and it’s shooting poorly you should probably replace it.

      That being said the pressures can be calculated throughout the total time a bullet is in the barrel most of the time being less than one millionth of a second. The powders burn rate, case capacity, bullet weight, seating depth, etc. all are variables. To fast or slow burning powders can cause excessive or not enough pressures. The best curve would be one where the peak pressure is achieved at the mid point of the bullet barrel time. This is what I believe the article is talking about.

      There is a great computer program called Quick Load. It will give you all the data related to a cartridge. All you have to do is input the data related to the bullet, or choice one from the list, input the powder, from the provided list and the charge weight and the program then calculates everything you ever wanted to know about that load. You can adjust all the data in the program including the powder data, that I would NEVER alter. Just remember when imputing data that junk in is junk out.

      1. Hi John,

        Thanks for the reply. Yep, I have Quick Load; it’s what made me ponder the question in the first place. It’s a great program.

        -Chris

  3. Great article. How does Nitriding/QPQ treatment weight in on the barrel life/accuracy on a chromemoly barrel?

  4. I know of a fellow who was shooting a 6.5 x 55 Swede and this elderly gentlemen could shoot 96 to 100 for the KNEELING position in 300 meter international match shooting.
    One day, another shooter brought his borescope along.
    Low and behold, the first TEN inches of his barrel had NO rifling left and still he was shooting HI Master / Distinguished shooter scores.
    It was a real privilege to be able to shoot with this man!

  5. I have been shooting a 6.5-06 AI for nearly 4 years, with approx 7000 140 grain bullets down the tube. I shot a .650 three shot with it last week.
    I have sworn to shoot it until it goes over moa

  6. I have at this time over 33,000 rounds of 147 gn’s. Bullets , loading my own with win 231, title group or bull’s-eye, all at about 3.4 to 4.2 gn’s. powder through a SIG p226- 4″ barrel and shoot every week about 400 + rounds, still holding it’s accuracy. I have all so put multiple thousands of same rounds through my 9 mm M4 , 18″ barrel and can hit 8″ steel targets at 100 yards every time. I do keep these weapons clean and well maintained after every range time . Would you say that the handguns and carbines shooting 9 mm are not affected as much because, I just don’t see any accuracy change at this point . What do you think about this ? I have also had a gunsmith inspect these barrels for any damage. I do have several buddies that are shooting tupperware’s guns as I call them, ill just let you guess what I am referring to buy calling them Tupperware guns, they have replaced some of their barrels because accuracy problems but then again I never in fact though they were shooting all that accurately in the first place.

    1. Hi jack
      not sure if anyone has replied to you. But I thought I would from my view point. You are really talking about apples and oranges here. Zediker is a competition shooter. Tho he says he is not a benchrest shooter he is still a precision shooter of the top tier. Rifle shooters are fussy, always going for the smallest groups possible and measure accuracy in tenths,hundredths or thousands of an inch at 200 to 300 yards plus. You sound like an IDPA or other action pistol sport shooter. I would say yes to 9mm being an entirely different animal. 147 gr titegroup loads run about 22k to 28k PSI pressure and a fast powder a small gas ball. Our pet load in IDPA for awhile was 3.3 grs of vihta 320 and a 147 gr bullet probably similar to your load. It is light and easy on the pistol. 223 will run up to about 55k psi. More if pushed by a competition shooter. 5.56 up to about 62,000 psi. most rifle people shoot sub moa groups at 100 yards. mostly impossible with a pistol (actually I would say completely impossible). my STI 9mm has about 20k rounds through it and shoots great. I believe you would probably have to be a pro shooter to shoot out a pistol barrel. I shot IDPA competitively for 5 yrs and have lately gotten into AR rifles and a bolt action 6.5 creedmoor. Its a whole different ball game from loading 45 and 9mm on Dillon 650. There is a lot more prep work and care to shoot 1-2 inch groups at 300 yards. Nature of the beast. Hope I didn’t come off as a know it all. Just been shooting for about 50 yrs and loading for about 40 yrs. I am just now learning about hi power rifles. I should have stuck with pistols, they are much easier. Also I have had to buy a bunch of new equipment.
      As a side note. My opinion of glocks was sketchy too until I started shooting a glock 34 in stock service pistol. Accuracy was excellent . Took me a couple of years to make master but it served me well. It was the stock barrel. Don’t forget, we are talking about combat accuracy. Please take this for what it is. I don’t know your area of expertise. have a good one.

  7. I need to clarify the point about “setting back” a barrel. It is true that a stainless steel barrel can be set back, so my statement wasn’t as much pure fact as it was recommendation. A few barrel makers have told me that it’s more difficult to get a smooth cut going on a stainless barrel unless it’s had more removed from its back end. Chromemoly machines a little easier. Make sure to start off with a barrel a good inch longer than what you might prefer to give room for setting it back later.

  8. Corrosion Resistant Steel, or CRES (“stainless”) barrels are _routinely_ set back by my gunsmith…

  9. Thanks for explaining in simple terms. There is a lot of information in this article concerning barrel life. SS vs Chromemoly is always a good discussion. For hunting guns seems wear and tear add up as much as shooting them out! Factor in cost of SS vs Chromemoly and you have another choice. Gosh so much fun figuring this out and so little time to do it! Again thanks for a basic informative article!

  10. “It’s clear, then, that lighter bullets will do less damage than heavier bullets, even though the lighter bullets mean burning more propellant. In a .223 Rem., for example, a steady diet of 77-grain bullets will shorten barrel life compared to using mostly 55-grain bullets. ”

    I dont understand the logic behind this statement that a heavier bullet will wear out a barrel faster. I am under the understanding that a barrel burns out and does not wear out as a normal practice. Using the lighter bullets in my understanding results in a higher peak pressure with more gases that erode the lands.

    In my experience it has been the lighter bullets with the heavier/faster powder charge that increases wear.

    Any thoughts?

  11. Great article. What might be a good follow up to this and Throat Erosion, Part II would be an article on specific methods for cleaning the throat of the rifle primarily.

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