Bore Capacity versus Barrel Life

[coyotezapper]

As I don't know the details of your tests to get the 1/2" size, I cannot speculate why you get those barrel life results. Is that half inch an average, smallest, biggest, mean radius...? How many shots per group? How was the rifle aimed? What group size is your limit when it pukes?

I don't think any center fire barrel will consistently shoot any group size for one nor two thousand rounds. They tend to slowly loose accuracy. Rim fire barrels used by winners get replaced at 30 to 40 thousand rounds.

Whose transducer psi system did you use to get those pressure numbers?

These two were just examples of custom and factory bbls. I have other rifles that also go against this so called data but these are just the two I picked so that rules out the anomaly statement. The 6.5 referenced is not mine but a buddies. Mine has only a couple hundred rounds through it but they were both built by GAP with the same reamer and we use the same load data. The 1/2" MOA reference was on the large end as this rifle generally / still shoots sub 1/2" MOA. The STW was shot with a few different bullets through the years but AVERAGED .5" - .7" depending on the bullet. Barrel puked when groups opened to 1.25" with .250" throat wear. In some hunting scenarios this accuracy would still be acceptable so this is MY definition of a bbl being shot out. Oehler 43 used for pressures / velocities.

I would like to see a large number of samples to prove the data on the charts. And to try and say poor accuracy is 1/2" MOA for 3 shots is a joke. What is your definition, 5 shots, 10 shots per group? Then there is the question as to how many groups define the accuracy?? How much time between shots?? There is a lot of variables to all of these questions. First and foremost the rifles mentioned are hunting rifles with relatively thin bbls in most cases not heavy bbl bench guns.

Yes all bbls will slowly lose accuracy the more rounds that go down range but the graph says bbl LIFE meaning the bbl is done. But there again is losing 1/8" in group size meaning the bbl is done? Not in a hunting rifle.

 

[Bart B]

coyotezapper,

I mentioned in my first post that hunting rifles will have a bigger MOA to start with. And their barrel lives will be longer.

My formula is based on 10-shot (or more) group average increasing 50% as the limit for barrel life. Most from Sierra's data using rail guns. With yours going from 1/2 inch to over an inch is a 100% increase. You can use whatever degradatioin standard you want for your objectives. It's based on rail gun and machine rested rifles for the most part so human nonrepeatability in holding and firing will increase group sizes.

Poor accuracy for everyone is when the bullets start striking too far from the point of aim for the shooters objectives. Top ranked rifle competitors have smaller limit numbers than hunters. Combat riflemen have the biggest numbers for service rifles and ammo. Competitive .308 Win rifle shooters rebarrel when their 600 yard test groups go from about 3 inches (maximum) to about 5. Arsenal 7.62 NATO M80 service ammo specs at 600 yards are about 20 inches (5 inch mean radius) and shoots about 30 inches in service rifles with new barrels; they're rebarreled at about 11,000 rounds.

You didn't mention how the rifle was held aiming it at the target shooting groups. Hand held against your shoulder resting atop bags on a bench? On bags in free recoil untouched by you except a finger on the trigger"

 

[coyotezapper]

Ok, here is another example for you. I just came across an article that Todd Hodnett wrote on barrel twists and he gave an example of a .308 Win that had a 1-7.8" twist bbl and it went 14,000 rounds before being shot out. He does not specify what his definition of shot out was but he did state the accuracy was 3/4" 5 shots ( which he categorized as ok ) until about 200 rounds where it became a 1/4" or better shooter. What caught my attention was not just the round count but the twist rate. Faster twist equals higher pressures. It was not clear on the manufacturer of the bbl but he did mention that only Rock and Broughton would make him a bbl with this twist rate at that time. So I am assuming cut rifle 5R from Rock and Button 5C from Broughton.

 

[Jaxdialation]

Thanks for posting this Bart.

The numbers seem relatively meaningful at the very least.
Sadly it is confirmation that my 243AI barrel is in the last 1/3
of its life

 

[Bart B]

coyotezapper,

Someone's comments on Sniper magazine article in 6/13 by Todd Hodnett. "...has been doing testing on twist rate for quite some time now, granted it has been with 338LM and the 308. ...he has found that buy going faster the more accurate a gun can be at long range. .... referring to his test using a 308 168gr factory match grade ammunition and getting consistent hits with a "14-inch barrel and an 1:8 twist. It had no problem hitting the target multiple times at a mile"

"He also addressed the concerns about barrel life and spinning jackets off. Some have theorized the faster twist would shorten barrel life. According to Hodnett his 308 with a 1:7.8 twist had 14,000 before he changed the barrel. Oh and he is not a believer of removing all the copper from the barrel after each session. In fact he won't do a copper removal at all unless he sees accuracy starting to diminish."

Zero info's provided to qualify those remarks about accuracy. To say nothing of the fact the ammo's going to have about a 36 inch drift at a mile with a constant 1 mph cross wind throughout its trajectory; that never happens. What about the target size? I oft times apply the "GB" standard to such claims. A Greyhound Bus has an area 31,000 times bigger than a Golf Ball; where in that bandwidth was his target size? And how many out of a Gross of Bullets (144 of 'em) will hit the desired target size?

And those bullets go subsonic at about 1000 yards.

What's the angle of the rifling to bullets in a 30 caliber 1:7.8 twist barrel? How does it compare to the same twist in a 22 caliber barrel?

Such claims are impressive to some folks. Not me. Hodnett provides no conditions and standards whatsoever for his claims to be based on. Sheepskin marketing; he's pulling the wool over someone's eyes.

 

[Rich Coyle]

coyotezapper,
Such claims are impressive to some folks. Not me. Hodnett provides no conditions and standards whatsoever for his claims to be based on. Sheepskin marketing; he's pulling the wool over someone's eyes.

Maybe his own.

 

[RockyMtnMT]

Does engraving pressure have any play in barrel life? We are making drive band type bullets that will have less engraving pressure that other bullets. Some manufactures of similar bullets to ours claim huge increase in barrel life. I am not willing to make that claim, as I am not sure that it is valid. I would like to test this theory, although I am not sure that I have the ability to do so accurately.

Steve

 

[TheFishBox]

http://www.longrangehunting.com/for...ore-chartidges-short-barrel-life-myths-61568/
Not sure if I posted this link right but this is worth a read when it comes to hunting rifles and barrel life.

 

[Trickymissfit]

no doubt you could get 14K rounds out of a .308. Yet what I want to know (like some others here) is what kind of groups and also pressure variations were happening at 3,000 rounds verses 6,000 rounds and so on.

I also question the round count. I think the guy is not telling everything. Guess it's time to drag out a pair of hip waders.

gary

 

[shaughn]

I think at one time someone (Ackley?) had a slide rule that one could use to figure out expansion ratio.. Case capacity to bore size and using another formula based on powder type, burn rate, bullet, rate of fire,temp and a few other factors to get a rather good estimate of barrel life.

My father who had competed in various styles of rifle competition as a young man said he generally, changed out barrels at 3000 rounds and he used the 30/06-7.62N-300 Winchester Mag.

As he put it the barrels had plenty of life and acceptable accuracy left in them (minute of game) just not enough to continue on when money or prizes were on the table.

For hunting rifles I imagine the numbers could be closer to 5000 rounds or more, dependent on caliber (6.5/378 W is going to go thru barrels like a coon shucking corn no matter what you do), provided a modicum of barrel care, no rapid fire strings, careful cleaning as needed.

 

[Bart B]

Sierra's test barrels shoot a 10-shot group in about 1.5 minutes once every 10 to 15 minutes. They grab 10 bullets as they come out of the pointing die machine then seat them in charged cases. Finished bullets come out of that last manufacturing stage at about 80 per minute for the entire production run. .308 Win barrels testing 150 to 180 grain bullets last about 3000 rounds.

The cost of those match grade test barrels is part of their wholesale prices.

 

[Kevin Thomas]

Does bullet weight not play a part in this as well?

Pardon my joining the party late, but this one deserved an answer; it's a most emphatic YES! Bullet weight plays a significant role in barrel life, with heavier bullets being harder by far on the barrels. Case capacity, velocity, pressures and so on certainly play a part in the equation, but bullet weight in and of itself is a major factor in regards to accuracy life.

My own contention here is inertia, and the fact that it simply takes longer to begin moving the heavier bullet down the bore. During that time frame, as brief as the time spans we're talking about may be, the high temps and pressures have more time to wreak havoc on the critical leade/throat area. This is based on having shot out hundreds of barrels over the years, with the entire range of bullet weights in any given caliber.

 

[Greyfox]

Pardon my joining the party late, but this one deserved an answer; it's a most emphatic YES! Bullet weight plays a significant role in barrel life, with heavier bullets being harder by far on the barrels. Case capacity, velocity, pressures and so on certainly play a part in the equation, but bullet weight in and of itself is a major factor in regards to accuracy life.

My own contention here is inertia, and the fact that it simply takes longer to begin moving the heavier bullet down the bore. During that time frame, as brief as the time spans we're talking about may be, the high temps and pressures have more time to wreak havoc on the critical leade/throat area. This is based on having shot out hundreds of barrels over the years, with the entire range of bullet weights in any given caliber.

It is good to hear an explanation based on your personal experience Kevin. I think it has been a conventional wisdom that the generally lower velocity of a heavier bullet for a given cartridge would be easier on the barrel, and therefore result in a longer barrel life. Your explanation of longer heat exposure is one that I have not heard.

 

[Bart B]

Pardon my joining the party late.....

Kevin, thanks for showing up. 'Tis hard for me to remember all that good stuff Sierra's Martin Hull passed on to me decades ago. Should have written it down then. Also good stuff from Bo Clerke, Elmer Shook and Bill Stratmann back then.

I think it has been a conventional wisdom that the generally lower velocity of a heavier bullet for a given cartridge would be easier on the barrel, and therefore result in a longer barrel life.

Yes indeed. Hadn't thought of this for a while.

Except that the longer the bullet's bearing surface is for a given velocity and bullet diameter into the lands, the more time it spends wearing away the rifling using powder and primer residue as a lapping compound for each shot. At least by my thinking.