- March 15, 2016 at 10:48 pm #25912rockratParticipant
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Came across a Kreiger barrel, 1-13twist, 28″ long. Wanting to fit it to a Rem action. I have seen some talk about haveing X amount of rounds down the tube is good for break in. So , little late to ask, but how many rounds thru a 308 would be too many. Assuming the barrel was well taken care of.
I was planning on cutting off 1/2″ off the chamber end and re-threading/rechambering.
- March 16, 2016 at 1:52 am #25917ArtfulParticipant
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Barrels Lose Accuracy After a Few Seconds of Bullet-in-Barrel Time
Here’s a little known fact that may startle most readers, even experienced gunsmiths: your barrel wears out in a matter of seconds. The useful life of a typical match barrel, in terms of actual bullet-in-barrel time, is only a few seconds. How can that be, you ask? Well you need to look at the actual time that bullets spend traveling through the bore during the barrel’s useful life. (Hint: it’s not very long).
If a bullet flies at 3000 fps, it will pass through a 24â€³ (two-foot) barrel in 1/1500th of a second. If you have a useful barrel life of 3000 rounds, that would translate to just two seconds of actual bullet-in-barrel operating time.
Ah, but it’s not that simple. Your bullet starts at zero velocity and then accelerates as it passes through the bore, so the projectile’s average velocity is not the same as the 3000 fps muzzle velocity. So how long does a centerfire bullet (with 3000 fps MV) typically stay in the bore? The answer is about .002 seconds. This number was calculated by Varmint Al, who is a really smart engineer dude who worked at the Lawrence Livermore Laboratory, a government think tank that develops neutron bombs, fusion reactors and other simple stuff.
On his Barrel Tuner page, Varmint Al figured out that the amount of time a bullet spends in a barrel during firing is under .002 seconds. Al writes: “The approximate time that it takes a 3300 fps muzzle velocity bullet to exit the barrel, assuming a constant acceleration, is 0.0011 seconds. Actual exit times would be longer since the bullet is not under constant acceleration.”
We’ll use the .002 number for our calculations here, knowing that the exact number depends on barrel length and muzzle velocity. But .002 is a good average that errs, if anything, on the side of more barrel operating life rather than less.
Gone in Six Seconds. Want to Cry Now?
Six seconds. That’s how long your barrel actually functions (in terms of bullet-in-barrel shot time) before it “goes south.” Yes, we know some barrels last longer than 3000 rounds. On the other hand, plenty of .243 Win and 6.5-284 barrels lose accuracy in 1500 rounds or less. If your barrel loses accuracy at the 1500-round mark, then it only worked for three seconds! Of course, if you are shooting a “long-lived” .308 Win that goes 5000 rounds before losing accuracy, then you get a whopping TEN seconds of barrel life. Anyway you look at it, a rifle barrel has very little longevity, when you consider actual firing time.
People already lament the high cost of replacing barrels. Now that you know how short-lived barrels really are, you can complain even louder. Of course our analysis does give you even more of an excuse to buy a nice new Bartlein, Krieger, Shilen etc. barrel for that fine rifle of yours.
Competitive highpower shooters I know tell me.
.223 shooters expect about 10,000 rds from a barrel.
The .308 Winchester crowd expects 6,000 rds.
I have heard that the .243 Winchester and 7mm Magnums might be around 1500-2000 rds.
This of course has many variables, including cartridge capacity for bore size
“Overbore” Cartridges Defined by Formula
Can a Formula Provide a Useful Index Ranking of Overbore Cartridges?
Forum Member John L. has been intrigued by the question of “overbore” cartridges. People generally agree that overbore designs can be “barrel burners”, but is there a way to predict barrel life based on how radically a case is “overbore”? John notes that there is no generally accepted definition of “overbore”. Based on analyses of a wide variety of cartridges, John hoped to create a comparative index to determine whether a cartridge is more or less “overbore”. This, in turn, might help us predict barrel life and maybe even predict the cartridge’s accuracy potential.
John tells us: “I have read countless discussions about overbore cartridges for years. There seemed to be some widely accepted, general rules of thumb as to what makes a case ‘overbore’. In the simplest terms, a very big case pushing a relatively small diameter bullet is acknowledged as the classic overbore design. But it’s not just large powder capacity that creates an overbore situation — it is the relationship between powder capacity and barrel bore diameter. Looking at those two factors, we can express the ‘Overbore Index’ as a mathematical formula — the case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives us an Index which lets us compare various cartridge designs.”
OVERBORE INDEX Chart
Overbore Index Chart
So what do these numbers mean? John says: “My own conclusion from much reading and analysis is that cartridges with case volume to bore area ratio less than 900 are most likely easy on barrels and those greater than 1000 are hard on barrels.” John acknowledges, however, that these numbers are just for comparison purposes. One can’t simply use the Index number, by itself, to predict barrel life. For example, one cannot conclude that a 600 Index number cartridge will necessarily give twice the barrel life of a 1200 Index cartridge. However, John says, a lower index number “seems to be a good predictor of barrel life”.
John’s system, while not perfect, does give us a benchmark to compare various cartridge designs. If, for example, you’re trying to decide between a 6.5-284 and a 260 Remington, it makes sense to compare the “Overbore Index” number for both cartridges. Then, of course, you have to consider other factors such as powder type, pressure, velocity, bullet weight, and barrel hardness.
Overbore Cases and Accuracy
Barrel life may not be the only thing predicted by the ratio of powder capacity to bore cross-section area. John thinks that if we look at our most accurate cartridges, such as the 6 PPC, and 30 BR, there’s some indication that lower Index numbers are associated with greater inherent accuracy.
This is only a theory. John notes: “While I do not have the facilities to validate the hypothesis that the case capacity to bore area ratio is a good predictor of accuracy — along with other well-known factors — it seems to be one important factor.”
I have had to replace a M1 Garand (.30’06) barrel – but I’m currently running a takeoff “shot out” barrel that was actually shortened about 2 inches to clean up all of the original chamber, and it shoots good enough for me.
- March 16, 2016 at 2:00 am #25918HarterParticipant
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The Army did a test with 30-06 stating that barrel life was 30 seconds or 10,000 rounds . I think it is well covered above .
- March 16, 2016 at 3:25 am #25920GoodsteelKeymaster
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Really doesn’t matter how “well taken care of” it is. Nothing is as harsh as the act of pulling the trigger on a 50,000 PSI cartridge. This boils down to round count and the style of cartridges that were used.
Being that its a 1-13, your round count just got a boost on account of the slow twist itself.
Being that its a 1-13, your round count gets a second boost on account of the probability that the 155s and 168s were what was going down the pipe (lighter bullets are better).
You don’t say, but I assume its a SS barrel which helps a little bit.
You say it’s 28″ long, which means you can knock 2″ off and set back which helps.
All things considered, if the guy selling the barrel says there are less than 5000 rounds through it, I’d be on that like bugs on a bumper.
- March 16, 2016 at 6:11 pm #25928RegParticipant
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Good post. Have never looked at barrel life in these terms.
- March 18, 2016 at 1:07 am #25964rockratParticipant
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Thanks, it is supposed to have just shy of 3000 down the tube. Figured for what the fellow was asking for it, I could take a chance. It is a stainless barrel
- March 18, 2016 at 11:35 am #25979GoodsteelKeymaster
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rockrat;n4416 wrote: Thanks, it is supposed to have just shy of 3000 down the tube. Figured for what the fellow was asking for it, I could take a chance. It is a stainless barrel
On second thought, that’s a really bad deal and you should run. However, I need to be aware of who this guy is so I don’t accidentally buy from him. What’s his number again?
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