Barrel Life: Powder Charge, Bore Size, Velocity and ???

[Phil3]

It seems to me, from what I have read and just common sense, that barrel life is probably largely determined by the amount of powder burned vs bore diameter. Consider the following. I used estimated averages for powder charges and values found on 6mmbr for bullet diameters. The area per bullet is in square millimeters and the final value (PG/SMM) is Powder Grains/Square MillMeter)

308 Winchester: 45 Grains. PG/SMM = .93
223 Remington: 25 Grains. PG/SMM = .98
6mmBR: 31 Grains. PG/SMM = 1.03
6.5 x 284: 50 Grains. PG/SMM = 1.41

The differences between the 308 and next two (223 and 6mmBR) are not as great as expected, but the well known short lived 6.5 x 284 is way out on front with the amount of powder behind its 6.5 bullet.

What else may be contributing to bore life? Does a smaller or larger bore just have an inherent advantage or disadvantage? What about velocity? Since most wear is in the throat, I would not think velocity is much of an issue, but perhaps chosen powders would affect things. If a powder burns a long time, the blow torch effect in the chamber and the gun may be longer, contributing to shorter life. Just speculating. Or is a quick high pressure burn worse for wear? And what about barrel length? If a bullet accelerates from 0 - 2800 ft/sec over a 20" barrel, that means the bullet (and hot gases) are in the barrel for .0012 seconds. If the barrel is 30" long, the time is roughly 50% longer. I know it would be less than 50% because the bullet is probably going faster, but for this content, the 50% value is sufficiently accurate, IMHO.

Amazing to think of the life of a 6.5 x 284 30" barrel measured in time, if the muzzle velocity is 2950 fps. The barrel life with bullet running down the bore over 1200 rounds is 2 seconds.

Phil

 

[butchlambert]

Phil,
What is your point? I think most of us has been down this road a few times in the last 20-30 years. We have different reasons for picking different calibers. Do I want one that last for ever or do I want one to win matches?

 

[Phil3]

What is your point?

The 4th paragraph that has questions.

I think most of us has been down this road a few times in the last 20-30 years. We have different reasons for picking different calibers. Do I want one that last for ever or do I want one to win matches?

I don't have the experience of 20 - 30 years and trying to learn more, but if that is not welcome here, then lesson learned with regards to future posts. I've no doubt everyone has different reasons for picking different calibers. Mine personally does not include matches.

Phil

 

[fdshuster]

phil3: In addition to powder versus bore diameter, and rate of fire (like a lot of rapid fire), I've seen proof positive with my Hawkeye borescope that a long, heavyweight bullet will erode the throat faster than a short, lightweight one.

I have an older Shilen barreled 6BR, 14 twist that has seen nothing but 30 grs. of N133 with the 68 gr. Berger #24411, presently with a total of 1816 rounds fired. I keep a count of rounds fired in the log book for each barrel. Just borescoped that barrel this morning after a thorough cleaning & there is not the slightest trace of firecracking in front of the chamber. Would it not be for a tiny bit of copper & carbon fouling ( I never clean down to bare metal, don't want the always required fouler shots before the barrel starts to shoot again), the bore could not be told from new.

On the other hand, my 6BR barrels that are 8 twist, both present and past, get a steady feeding of 30 grs. of Varget with bullet weights from the 105 to 108 Bergers, with some 107 SMK's. Every one of these barrels have the beginning of firecracking by as little as 850 rounds or as much as 1000. None of these barrels have ever lasted for more than 2000 rounds. By then they are starting to throw uncalled flyers, cannot be depended on in a match, and all exhibit heavy copper fouling.

The 6ppc barrels, all with 14 or 13.5 twists & using 28.5 grs. of N133 with 65 to 68 gr. Bergers show the same erosion (or lack of) as the 14 twist 6BR's, with similar rounds fired. Cannot say what a fast twist ppc barrel would look like with heavyweight bullets because I don't have any.

So, for me, add heavyweight, long bullets as also contributing to throat erosion. Barrels are an expendable item, like primers, powder, bullets, etc. so I don't worry about it, just replace them when they're done.

 

[firearmdoc1]

Phil,
What is your point? I think most of us has been down this road a few times in the last 20-30 years. We have different reasons for picking different calibers. Do I want one that last for ever or do I want one to win matches?

Phil,
I don't have any answers for you regarding your questions, but don't let some BS reply like this keep you from asking questions.

 

[butchlambert]

So how do you decide my post was BS? If you didn't like it fine, but how is it BS?

 

[Steve Blair]

What else may be contributing to bore life? Does a smaller or larger bore just have an inherent advantage or disadvantage? What about velocity? Since most wear is in the throat, I would not think velocity is much of an issue, but perhaps chosen powders would affect things. If a powder burns a long time, the blow torch effect in the chamber and the gun may be longer, contributing to shorter life. Just speculating. Or is a quick high pressure burn worse for wear? And what about barrel length? If a bullet accelerates from 0 - 2800 ft/sec over a 20" barrel, that means the bullet (and hot gases) are in the barrel for .0012 seconds. If the barrel is 30" long, the time is roughly 50% longer. I know it would be less than 50% because the bullet is probably going faster, but for this content, the 50% value is sufficiently accurate, IMHO.

[br]
Heat is what kills barrels. So, how much heat applied for how long is the determinant. Some powders have higher heat potential than others. Higher pressure means more heat. Burning more powder applies more heat to the barrel. Unfortunately, there is no free lunch. If you want high performance, you have to burn powder. That will reach a point of diminishing returns where the gain is not worth the price. [br]
As caliber increases, the ballistic coefficient of available bullets generally increases as does the amount of powder necessary to drive them. Obtaining high velocity from any caliber translates to pressure over time. Again, more heat into the barrel. The barrel length is irrelevant to throat erosion. The area of the throat that suffers is heated much faster than the ability of steel to carry away the heat and distribute it through the barrel. Sometimes, a barrel is done when only about two inches are badly eroded. [br]
As Frank said, keep an eye on your barrel and scores and replace it when it is done. I order two new barrels for a given rifle when I have two chambered. Works out about right.

 

[BoydAllen]

Another factor is the flame temperature of various powders. A cooler burning powder of the same general burning rate, will generally give longer barrel life, all other things being equal, than one that has a hotter flame temp. There is also the consideration of how powders foul. If you have to resort to abrasives on a fairly regular basis, to stay ahead of a given powder's fouling, you may see more cleaning related damage than if you are shooting a relatively clean powder that does not require cleaning as often or by methods that are as likely to cause damage.

 

[mao0720]

Boyd, is there a published chart of burn temperatures for different powders?

 

[BoydAllen]

I have seen figures quoted, but I do not know the source. Generally double base powders burn hotter than single base, because of their nitroglycerine content. Beyond that I have heard that Winchester 748 is cooler burning than other powders of similar burning rate. I have also heard the same thing about H1000. Perhaps some others will chime in on this. Back in the day, a very experience highpower shooter that Knew all of the best shooters in the state, and was one of them, told me that in the magnum prone rifles that were common at the time, that it was pretty commonly known that some powders gave longer barrel life than others. Questions like this are best asked of shooters that put a lot of rounds down range, who compete, and share their experiences with other competitors.

 

[Greyfox]

Butch asked a good question. If you're looking at a deer rifle, you may only fire it 3-4 times in a season. It will last several generations. If we're talking about prairie dogs it could be 500 in a day ( I wish). A 6PPC might be past it's prime in a thousand rounds and a 30BR could do 6-7000 for score shooting and still win matches.

I've thought about the actual time a bullet spends in the bore and come up with the same conclusion. It ain't very long. So why are you asking? And what are you looking for. It's not BS, it's just difficult to answer the question if we don't know where you're trying to go. Barrels are like bullets, powder and primers....expendable.

Rick

 

[Phil3]

So why are you asking? And what are you looking for. It's not BS, it's just difficult to answer the question if we don't know where you're trying to go.

Simply out of curiosity. I like to learn about firearms, and not always searching for an answer to a particular problem. I was not the one that called "BS".

Phil

 

[Erik Cortina]

N165 is one of the coolest burning powders there are.

There is a spreadsheet that tells you how many accurate rounds to expect out of a barrel with a given powder charge, pressure, and powder hear potential. Do a search on the daily bulletin and you should find it.

I think of barrels like race car tires.

 

[Phil3]

Thankfully the barrels last longer than a weekend.

Phil

 

[Erik Cortina]

Thankfully the barrels last longer than a weekend.

Phil

Depends on your application, some don't.

 

[Phil3]

Nor do some of the tires. - Phil

 

[Nvreloader]

Doesn't QL have this in it's program, called Heat of Explosion/Potential???

The listed info is for each powder is listed, from Bullseye @ 5180 to N32C Tin Star @ 3040 kj/kg.

Tia,
Don

 

[gotcha]

Interesting. Mikes variables don't seem to include bearing surface contact. Or, did I miss something??

 

[BoydAllen]

Friction is not what wears out barrels, a very thin hard, brittle layer forms where the heat and pressure are the greatest, and it cracks under pressure, and then starts to erode starting at the cracks. If friction was the culprit, they would wear near the muzzle first.

 

[gotcha]

Thanks Boyd. That's why I come here. Another old wives tale dispelled.