From: Bart Bobbitt <firstname.lastname@example.org> Subject: Calculating Rifle Barrel Life I finally completed my research on a way to calculate how many rounds a rifle barrel can be expected to deliver its accuracy level. By that, I mean the barrel can be expected to have an average group size for so many rounds before that average group size starts to get larger. First, the rule-of-thumb formula I derived will produce a barrel accuracy life of about 3000 rounds. Second, if a lot of rapid fire (one shot every 5 to 10 seconds) is done, the accuracy life will be less. Third, if full-auto or very fast fire (a few shots per second, or one shot every second) is done, accuracy life will be much less. Fourth, the accuracy levels are for ranges through 600 yards. Once the barrel life calculated limit is reached, groups will probably start to get bigger at the longer ranges before they are noticeably bigger at the shorter ranges. My formula, or rule-of-thumb process, is: 1. Calculate the bore area in square millimeters. 2. Use one grain of powder for each square millimeter. This is what I call the reference, or base powder charge. Example: .30 caliber bore = 45.6 square millimeters. Base powder charge for .30 caliber is 45.6 grains. A .30 cal. cartridge that burns 45.6 grains of powder should give a barrel life of about 3000 rounds of good accuracy. 3. If a larger cartridge is used and it burns more powder, the accuracy life in rounds for that bore size is reduced. The amount of reduction is determined by a. Divide the increased charge by the base charge, then square the answer. b. Divide that answer into 3000. Example: .28 caliber bore has a base charge of 38.5 grains. Cartridge burns 57.8 grains of powder. (57.8/38.5) squared is 2.25. 3000 divided by 2.25 is 1,333 rounds. If anyone can shoot a hole through this theory, I welcome that shot. This is more or less an emperical process based on accurate barrel life in several calibers as reported to me by lots of folks. All I did was study the data and determine what math would give a best-fit formula. And if someone has a better method, I'd like to know what it is. My formulas may not be the best.
From: email@example.com (Bart Bobbitt) Subject: Re: Throat Erosion?? Organization: Hewlett-Packard Fort Collins Site william E Davidsen (firstname.lastname@example.org) wrote: : I have a friend who shoots 1000yd matches, and I'm pretty sure he : quoted his barrel life as 200 rds. He's given me a few nice top name : barrels, but they are no longer accurate and need to be bored to a : larger caliber to restore accuracy. : Therefore I would conclude that the useful accurate life depends on : how well you and the gun shoot as a starting point. Well, I've not heard of anyone getting that short of life out of a barrel. I knew a couple of folks who used the 6.5 X .300 Weatherby for a 1000-yd rifle and they got about 500 rounds of accurate life. But I agree with your comment about how well someone shoots as well as the gun shoots is a big factor in deciding when to retire a barrel. If someone doesn't notice his system (person, rifle and ammo) accuracy has got worse, they probably won't think their barrel has worn out even though it really has. For example, a benchrester gets a new rifle barrel and it shoots no worse than .250 inches in good conditions at 100 yards; many groups go under .150 inch. When that barrel starts shooting groups in the .300 to 400 inch size at 100 yards, he gets a new barrel. With the .22 and 6mm PPC cartridges these folks use, this drop off in accuracy typically starts at about 3000 or so rounds. Should an average varmit hunter's rifle start shooting groups a tenth of an inch larger at some point in its barrel life, he probably won't notice it and continue to use that barrel for many hundred more rounds, perhaps a thousand or more. His acceptable level of accuracy may not be as stringent as the benchresters. My comments regarding barrel life are relative to when accuracy starts to degrade. Whether or not the shooter notices it, or even cares about it, is an entirely different matter. BB
From: email@example.com (Bart Bobbitt) Subject: Re: Barrel wear on M-16 or Beretta 92F Organization: Hewlett-Packard Fort Collins Site Richard Cower (cower@CSLI.Stanford.EDU) wrote: : Does anyone know how many rounds can be fired thru the Colt M-16 : before the barrel is worn out? The barrel should maintain its initial accuracy level for about 3000 rounds when shots are fired no faster than about 1 per minute. Then its accuracy will start getting worse, although it may not be noticed. For plinking or hunting use, it could last for about 5000 rounds; it depends on what your accuracy requirements are. If a lot of rapid firing is done, like shooting at a rate of 10 or more rounds per minute, and doing this a lot, the barrel life will be shorter by as much as 50 percent. BB
From: firstname.lastname@example.org (Bart Bobbitt) Subject: Re: Barrel wear on M-16 or Beretta 92F Organization: Hewlett-Packard Fort Collins Site Aaron Good (ucla-cs!netcon.smc.edu!agood@CS.UCLA.EDU) wrote about my comments on .223 barrel life: : I have trouble with these numbers, they seem awful low, are you sure ? : IMHO that many rounds would be fired just in basic training. Are you : talking about match grade barrels or something ?. I'm talking about any barrel. Match and sporter barrels wear at the same rate; they're made from about the same materials, only the uniformity of the bore/groove/twist dimensions are different. As the user gets to decide what the `real' barrel life is, each user's number may be different. But the point at which the initial accuracy level starts to open up is the basis for my numbers. Some folks will accept the groups getting a lot bigger than others after this point is reached. I've seen some .308 Win. barrels that have had more than 15,000 rounds through 'em. For the first 3000 rounds, they would shoot better than half MOA through 200 yards. At 15,000 rounds, they shot 3 MOA groups through 200 yards; for some folks, that may be acceptable. BB
From: email@example.com (Bart Bobbitt) Subject: Re: Barrel wear on M-16 or Beretta 92F Organization: Hewlett-Packard Fort Collins Site Richard Cower (cower@CSLI.Stanford.EDU) wrote: : Does anyone know how many rounds can be fired thru the ........ : Beretta 92F before the barrel is worn out? How about the Glock : 17 or 19? My barrel wear-out numbers were for rifles; not handguns. Please don't mix or confuse the two; handgun barrels last much, much longer; they burn much less powder. My only real knowledge about a handgun barrel's life is based on a test the US Navy did in the late 1960s. An accurized M1911-A1 in .45 ACP, a softball (wadcutter) competition pistol, was built and then put in a machine rest for its final tuning and initial accuracy level. It started out with 20-shot 2.5-inch groups at 50 yards. About every 5000 rounds, it was put back into the machine rest and retested. At each of these tests through 25,000 rounds, it still shot the same size groups. At 30,000 rounds, the groups had opened up to about 3.5 inches; at 35,000 rounds, the groups were about 4.5 or so inches. The ammo used was Remington 185-gr. jacketed wadcutter match ammo as loaded by Remington. The top pistol shooters on the team had been wondering just how many rounds a softball gun would last before it needed rebarreling. They knew the hardball match pistols (using full-power 230-gr. loads) went for about 15,000 rounds before needing a new barrel. By this, I think it means that the initial accuracy level of the barrel was good for about 15,000 rounds and it started to open up after that. Again, this is where accuracy starts to drop off, but for purposes other than pistol competition, the user gets to decide when the barrel is worn out. Interestingly enough, both handguns and rifles for the .22 rimfire round last about the same number of rounds. That's expected, as the both use the same amount of powder. These rimfire barrels go for about 5,000 to 10,000 rounds before accuracy starts to drop off. As their barrels are typically made from steel that's somewhat softer; they wear out faster. I know there are some .22LR rimfires around with 50,000+ round through them that their owner's think their accuracy is just fine; and that's just fine, too.
From: firstname.lastname@example.org (Bart Bobbitt) Subject: Re: Barrel wear on M-16 or Beretta 92F Organization: Hewlett-Packard Fort Collins Site Quoc Tuan Pham (email@example.com) wrote: : For the Anschutz rifle, it takes about 500 : rounds to "break in" the barrel (so it becomes smooth and does not lead : up easily). Serious shooters usually shoot 15,000-20,000 rounds a year : for training and some of them are very fussy about the accuracy. So : probably these barrels don't wear out that fast as said in other : postings for center-fire barrels. In talking with some of the folks at the Olympic training center and other top smallbore competitors in the USA, your rounds-per-year numbers are about what smallbore barrels used to get for top accuracy. But that was before RWS and Ely changed their priming compound formula to be more `environmentally correct.' For about the last 5 years, barrel life of a smallbore match rifle got cut about in half. Seems the new priming compound causes more (faster?) throat erosion. Interestingly enough, there's a bunch of new Anschutz barrels stored in closets by 'smiths catering to the smallbore competitors. Seems there is two reasons for this: Hart barrels shoot more accurately and they last longer for accuracy. One smallbore 'smith told me a few years ago that the Anschutz factory barrels were a tad softer than a Hart and that's why they didn't last as long. BB
From: firstname.lastname@example.org (Bart Bobbitt) Subject: Re: Barrel wear on M-16 or Beretta 92F Organization: Hewlett-Packard Fort Collins Site Clark Towle Gunsmith (email@example.com) wrote: : Can't say I've ever seen a Beretta 92F barrel get worn out but an M16 barrel : in the 1/7 twist can go south in as little as 2,000 rounds depending on the : rate of fire it is subjected to. Dumping several 30 round magazines in a row : without allowing the barrel time to cool off creates a premature throat : erosion condition which only gets worse with time. Amen to that! Several years ago, one of the military rifle teams got a bunch of .300 magnums rebarreled with match-grade tubes. One of the things going around at the time was that these barrels needed to be broken in by shooting them. Seems that gave a case of ammo to someone to take out to the pistol range and shoot several rounds through each barrel. Well, the well-meaning person picked up a rifle, chambered a round pointed it into the backstop and pulled the trigger. Then lowered the rifle immediately, chambered another round and fired it. This process was repeated for about 50 or 60 rounds per barrel; all shots for each rifle being fired in about 4 minutes. Three of these rifles were used in a match the following weekend, but without any great success. The folks who shot them said they all just didn't shoot as well as expected, and perhaps the ammo wasn't too good as it was a new, just loaded lot. The following week these rifles were taken out to the local range and tested with the remnants of the last lot of known good ammo; they shot about like a barrel that had a thousand rounds through it. The other three rifles showed the same thing. Then someone remembered the break-in process and suggested the person who broke in the rifles be questioned. Well, after the questioning, everybody knew why the rifles didn't shoot well; they had been burned out through break in. The match barrels used weren't as hard as a machine gun barrel; that's why a 60-round rapid fire string wore them out for competitive purposes. BB
From: firstname.lastname@example.org (Bart Bobbitt) Subject: Re: Barrel Life During Combat Conditions Organization: Hewlett-Packard Fort Collins Site Bill Vojak (email@example.com) wrote: : This made me wonder; In actual COMBAT situations, how often does the military : change the barrels. It would seem that a soldier in combat could put lots : of rounds through a barrel in a VERY brief period of time. Does the military : just say "oh well, so what if the 100 yard groups are now 5"", or do they have : some standard for replacement. I would think that during Vietnam some of : those rifles had 10s of thousands of rounds put through them over the years, : many while the rifle was in a burst or full auto mode. Our armed forces do have a way of determining when a barrel should be replaced. But even then, when that time is reached, it should be somewhat convenient for the person who has a worn out barrel. A throat erosion gage is used. It has a tapered, hardened steel plug on the end that goes into the throat. Taper is typically .010-in. per inch. for 30 caliber arms, such as the 7.62mm NATO, the gage will go into the throat so far as to have the number 0 on its body next to a reference point in the receiver. If the gage goes in to where the number 5 is next to the reference mark, that means the bore diameter has opened up about 005-in. at the origin of the rifling. Usually, this means the barrel is about half way worn out. If the gage goes all the way in to where the `reject' point on the gage is next to the reference mark, that means the barrel should be replaced. Military armourers who work on match grade service rifles usually rebarrel when the gage reads 5. That's the point where accuracy starts to drop off from the level it had as a new barrel. In talking with military folks who rebarreled a lot of service rifles, the number of rounds varied quite a bit amongst barrels for a given number read off the throat erosion gage. Sometimes, if the barrel was used for a lot of rapid fire, the barrel might reach the `reject' point in about 3000 rounds. If mostly slow fire was done, the barrel might go for close to 10,000 rounds. Machine guns are noted for wearing out barrels quite fast. Most machine guns are built so barrels can be changed very quickly; makes sense when they're in a combat situation. As arms used for service/combat applications don't need to be as accurate as a competition target arm, they typically last a few more times as many rounds. Besides, the skill level most service folks have, and the environment they're in, doens't require half MOA accuracy for the life of the barrel. I doubt if `tens of thousands' of rounds per service barrel is even closely approached. With the extreme amounts of throat erosion that many rounds would cause, the reduced pressure levels would probably not be enough to cycle a semiauto or full auto action. In talking with Vets from WWII, Korea, Vietnam, few fired more than 4000 to 5000 rounds in combat with semiautos. But the machine gunners did; and also replaced a lot of barrels. With the machine gunners, they said they could tell when the barrel needed to be replaced; tracers started going in all directions. They didn't need a throat erosion gage; just a few minutes to replace the barrel. BB
From: firstname.lastname@example.org (Bart Bobbitt) Subject: Re: Barrels and rapid fire Organization: Hewlett-Packard Fort Collins Site Rapid fire does wear out barrels much faster than shooting at a slower pace. The typical rifle in .308 Win. will keep its original accuracy for about 3500 rounds when fired not faster than about 1 shot per minute. If 30% of the rounds fired are rapid fire, that is one shot every 4 to 5 seconds, the barrel life will be about 3000 rounds. If 60% of the shots are rapid fire at the same rate, barrel life will be about 2500 rounds. And if 90% are rapid fire, expect only about 2000 rounds of original accuracy. If the powder charge is larger for this same bore size, the barrel life will be shorter; for example a .300 Win. Mag. will last about 1100 rounds of original accuracy when shot no faster than once per minute. But rapid fire will wear one out in only a few hundred shots. Increasing the rapid fire rate to one shot every 1 to 2 seconds will reduce the above shorter barrel life increments by about 50%. In other words, the 30-60-90 percent of rapid fire cuts the accuracy life to 2750, 2000 and 1250 rounds, respectively. These numbers are for type 416R stainless steel barrels, the type most used for competitive shooting which is what my numbers are based on. Machine gun barrels are typically much harder (but not as accurate) as barrels used for sporting purposes. They'll last much longer; perhaps three times longer. But replacement barrels are usually nearby for those combat-environment machine guns and they can be replaced in a very short time. Some years ago, one of the military rifle team armorer's sent 5 brand new 30-.338 magnum target rifles out with someone to `break them in.' That meant they should be shot, then cleaned, then shot and cleaned again for about 20 shots. That's at a rate of about one shot every 5 minutes or thereabouts. Well, seems the `shooter' was on the pistol team and he just didn't understand all of this. He shot each one about 30 times at a rate of one shot every 5 seconds; just about as fast as he could shoot, reload and shoot again. All five of those rifles were taken to a 1000 yard match the following weekend. None of them shot accurately. Expecting them to produce groups at 1000 yards of about 7 to 8 inches, the best of them did only about 1.5-foot groups; the worst about 3-foot groups. Needless to say, the managing armorer was angry enough to chew those ruined barrels into a pile of stainless steel wool. BB
From: email@example.com (Bart Bobbitt) Subject: Re: Competition small bore [.22]. Organization: Hewlett-Packard Fort Collins Site Geoff Miller (firstname.lastname@example.org) wrote: : ...a well-maintained smallbore target rifle is almost impossible to : wear out (unless you're doing as much practice as an international- : class shot). Prior to about 1988, one could get 20,000 rounds of accurate life from a .22 LR target barrel. But RWS R50 and Eley Tenex (Lapua Dominator, too, I think) changed their priming compound to a more environmentally compatible one. In doing this, the increased amount or addition of one component in the priming mixture is now more abrasive to the barrel steel. Nowadays, one gets about 10,000 rounds of accurate barrel life. That means one can shoot about thirty 3200-point matches with some sighters included and no practice shots at all. Then, you need to either set the barrel back a couple of inches or replace it if you want the same level of accuracy in your mouse gun to be maintained. I've heard the M52 will be back in production, but with hammer-forged barrels. 'Twill be interesting to see how well they shoot. The original M52D cut-rifled barrels were pretty good; about one-third of them shot as well as a Hart barrel. But they needed to be set back and rechambered with a quality match reamer; factory chambers were a tad big and irregular. If the reintroduced M52s are well made, this may be a boone to the .22 LR competition folks getting started in the smallbore discipline. BB
From: email@example.com (Bart Bobbitt) Subject: Re: ACCURACY? Organization: Hewlett-Packard Fort Collins Site David Post (firstname.lastname@example.org) wrote: : And there was the discussion of copper jacket versus steel jacket ... the : conclusion of which seemed to be that the jacket metal did not matter. You : see, I am having trouble seeing why the different metals have similar wear : properties. My use of soft-steel jacketed bullets (Norma) and other match bullets shows equal barrel life with both, for the same caliber. Steel jacket material Norma uses has about the same hardness and other properties as gilding metal used in most bullet jacket material. : But now we are talking about barrel hardness affecting useful barrel life. : While it seems reasonable, I'm having trouble rationalizing it with the : other discussions. Stainless steel used for rifle barrels varies somewhat in its metalurgy and hardening processes, as well as machinability. With softer stainless steel, button rifling can make much more uniform groove diameters; especially when there's a few non-homogenous places in the barrel blank. Oft times, no bore lapping is needed to get bore/groove dimensions to within .0001-in. or less of desired dimensions. This means the barrel costs less for the customer; a very common thing with some barrels. Harder stainless steels require some lapping of the button or cut rifled bore to get very uniform bore and groove dimensions. That's more work for the barrel maker, so he wants paid for his extra labor. That's fine, as the harder barrels (Hart, Krieger, Medesha and Obermeyer) last longer. When you divide the cost of having a barrel installed by the number of shots of accurate life, the harder stainless steel barrels that last longer also cost less per shot for their life. Another cause of shorter barrel life is rifling groove depth. The better barrels in 30 caliber, for example, have a groove depth of 4 to 6 thousandths of an inch. For a given barrel metallurgy, barrels with grooves this deep will have an accuracy life about 40 to 60 percent more than one with grooves only 3 thousandths of an inch deep. Shallow grooves plus softer steel means a really short accurate barrel life. : And what is it about the above calibers that actually causes accelerated : wear and how does that relate to the mentioned discussions? It is not : intuitive to me, especially after comparing published max load velocities : for 165-170 gr bullets. ( Hodgdon lists ~3200, ~2800 and ~2600 : respectively). I've used the following rule of thumb for several years to track the accurate life of barrels shoot everything from PPC cases to very large magnum cases in 30 caliber. It's proved very accurate, at least within 10% of what folks get. My data is bases on what competitive shooters get for accurate barrel life. But hunters, plinkers, and other rifle shooters will get the same results; the barrel doesn't know the shooting conditions it's being used in. Whether or not the shooter-rifle-sights-ammo system can tell when accuracy starts to get worse by a quarter or third MOA is subject material for someone elses keyboard time. Rule 1. A centerfire cartridge whose maximum safe powder charge weight in grains is equal to the bore's cross sectional area in square millimeters will have about 3300 rounds of its initial accuracy maintained when fired shots are no faster than about one minute apart. Firing more rapid will shorten the barrel's accurate life. This is at bore capacity for that bore's diameter. Harder steels with groove depth 4 - 6 thousandths are used in this emperical extragavanza. Softer steel barrels or shallower groove depth will reduce barrel life by 20 to 50 percent. Bullets used are about .0005-in. greater in diameter than the barrel's groove diameter. For example: Bore Diameter -> .22 .24 .26 .28 .30 Bore Capacity in grains -> 24.3 28.3 33.2 38.5 45.6 Rule 2. Increase bore capacity powder charge by: 100%, barrel life shortens 75% 50%, barrel life shortens 67% 25%, barrel life shortens 33% BB