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Barrel Wear

 
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  #1  
Old 03-01-2011, 05:13 PM
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Barrel Wear

just curious if anyone can tell me if it's the pressure(higher pressure=higher burn temp) or the amount of powder(longer burn time) that wears out a barrell?



ie. would a 7mm ultra mag with a 26" barrel loaded at less than full pressure to achieve approx. 3000 feet per second with a round or a 7mm mag which uses half the powder loaded to full pressure have a longer barrel life?
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  #2  
Old 03-01-2011, 08:35 PM
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Re: Barrel Wear

Taz,

essentially, it's "X" pounds of powder. Whether that powder is divided into lots of smaller charges or fewer large charges, it still remains X pounds of powder. The trick is, this only applies when all other factors remain the same, and you have a host of other variables involved. Higher pressures do more damage, as do heavier bullets. Hotter powders (think double based powders with relatively high nitroglycerine content and higher flame temps) are also a factor.

In short, it's an equation with a number of different variable parameters, and the total of all of them is what will dictate the results. I see a lot of people who get concerned about barrel life, but that's the tradeoff. Speed costs money. How fast you wanna go?
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  #3  
Old 03-01-2011, 08:42 PM
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Re: Barrel Wear

I have thought about it a few times through the years, as I'm sure others have. Common sense (if it was "common" everyone would have it, right?) tells me more powder creates more heat, but more powder is usually associated with bigger cartridges and thus, probably bigger bores. I think large quantities of powder being burnt down small bores create the most damage to a barrel.
Example: if you shoot a 7mmRM with 51.0gn of IMR4320 (175gn Partition) and a .243Win with 52.5gn of H414 (55gn Ballistic Tip) seems the .243 would bite the dust first. There's not a lot of difference in the burn rate of these two powders and the Maximum Average Pressure (PSI) recommended by the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) is 61,000 for the 7Mag and 60,000 for the .243Win.
My first 7Mag Sendero's barrel went south with a little over 1200rds down the tube. I shot a few 120's and 140's but the majority of bullets were the 150gn with 67gn's of RL22.
Another factor I think hurts your barrel is excessive and sustained heat like one may encounter in a "target rich environment" (i.e. prairie dog town). Obviously not everyone has to worry with this so it may or may not be a concern.
Keep it cool and clean and it will probably last a while. JohnnyK.
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Old 03-02-2011, 08:24 AM
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Re: Barrel Wear

As I understand it (I'm an electrical engineer, not a metalurgical engineer) from reading and talking to some of my metalurgical engineer friends, the process that wears out the throat is a combination of thermal shock errosion and abrasion (from several things).

Any meaningful discussion has to begin with a description of the process that causes the erosion in the throat. Somewhat simplified, the blast of hot gas (above the melting point of the barrel metal) heats the metal on the surface of the throat and immediately beyond cherry red to a depth of a a thousandth of an inch or possible a bit more. The exposure to the gas doesn't last very long, milliseconds, and it is very hot (couple thousand degrees), and under high pressure. That combination heats just the surface of the metal. An eighth of an inch or even a sixteenth of an inch deep there is a temperature change but nothing like the several hundred degree change on the surface. It is this temperature gradient from cherry red on the surface to only a bit warmer a few thousandts of an inch deeper that casues the errosion.

The metal on the surface expands with the flash of heat, then cools very rapidly as the heat source goes away and it loses heat rapidly to the much much cooler amd more massive collection of barrel metal immediately adjacent to the hot layer. The differential expansion of the surface metal with respect to the immediately adjacent metal causes thermal stress, a shearing stress between the layers of metal. This stress cycle is repeated with each shot, and will eventually cause a thermal fatigue failure of the metal on the surface.

Metal will flake off having fatigued to the point that it is no longer attached to the metal underneath. The surface will start to look like it has scales. The surface metal expands and distorts itself with respect to the layer underneath, then shrinks which with enough repititions causes scales to form when it contracts. It will look almost like the surface of a dry lake bed when viewed through a borescope. I think I have some pictures of this at home that I've taken in my shop. I made a setup to use my Nikon 990 with the Hawkeye borescope to take pictures.

It's a cumulative thermal shock fatigue effect. The more damage done per round the shorter the barrel life.

The other mechanism is abrasion, wear, of the throat and barrel by material coming out of the cartridge - powder, bullet, burnt powder, and so forth. More of this causes more wear than less.



OK, with that basic understanding of the mechanism, one can draw some qualatative conclusions about what will make it worse or better:
  1. Anything that increases the rate of change of temperature on the surface will increase erosion. Hotter gas temperatures, faster temperature rise, will increase erosion per round fired. Faster powder will generally be hotter at the throat than slower powder. More powder will increase the depth of the hottest layer by keeping it hot a millisecond or two longer.
  2. Lighter bullets with fast powder may be harder on the throat than heavier for caliber bullets and slower powder.
  3. More powder hurts two ways - more material to grit blast the throat area as it moves into the barrel and sustained heat to increase the depth of the heat affected zone.
So the cartridges that would be expected to have shorter barrel life would be those that have a lot of powder relative to the bore volume because they increase the thermal shock and dump more grit into the throat. A 7 mmMag will be harder on the throat than a 7mm-08. A 7mmBR will be easier on the throat than a 7mm-08. The .308 will have longer barrel life than a 7mm-08 (but probably not a lot longer) because of the larger bore which allows the gas to get past the throat sooner with the same weight bullet. I'd expect a .338 Federal to last about forever, but maybe not as long as a .30BR.

I'm not at home right now, so I can't give you a link, but there are a couple of spreadsheets available on the WEB that proport to predict useful barrel life. Playing with them, at least qualatatively they show variations as a function of powder choice (burn rate), bullet weight, and powder charge weight, that are at least consistant with this theory - i.e. the changes in barrel life are in the direction expected based on the mechanism described.

A .30BR has amazing barrel life. If my memory is right, national matches have been won shooting barrels with 6,00 to 7,000 rounds through them. The relatively small powder charge, and relatively large bore volume compared to case volume is all in the right direction to promote barrel life and more than overcome the light for caliber bullets usually used in a .30BR. If you want a rifle to practice with that will shoot well for thousands of rounds, get a .30BR.

Anyhow, that's how I understand it. I have no indispensible ego attached to this so if you have a different theory, please put it out there so we can have a dialog about it.

Fitch
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Last edited by Fitch; 03-02-2011 at 08:28 AM.
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  #5  
Old 03-02-2011, 09:50 AM
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Re: Barrel Wear

Fitch,

A very reasonable and in-depth summary here. I alluded to the other variables, and you highlighted some of these. You obviously have a bore scope, as do I, but for those who've never used one, it's enlightening to say the least. All the action takes place in the first few inches of the bore, and in the first few milliseconds as the bullet begins to move. Not at all uncommon to find a throat that looks like a mile of really bad asphalt, while the remainder of the barrel looks like it's never seen a round fired.

You covered the flame temps, and I'd touched on the same. JohnnyK mentioned two powders with similar burn rates (and he's perfectly correct there), but didn't mention that the IMR is a single based propellant, while the H414 is double based powder. This is one of the differences I highlighted in pointing out that it's a sliding scale with a number of variables to be considered.

This also goes a long way in explaining why heavy bullets are so much harder on barrels (specifically, the throats) than lighter ones. Longer dwell time, as the heavier bullet has more inertia to overcome before it begins to move, gives those hot gasses and high pressures a good bit more time to allow them to work their damage. Once they begin moving, the rest of the bore is comparatively undamaged by the bullet's passage.

Good write up.
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  #6  
Old 03-02-2011, 11:45 AM
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Re: Barrel Wear

A barrel can 'die' in multiple ways..
Even if performance extends past throat erosion(which is common), there is an ultimate killer shortly to follow: Carbon restriction
I don't know the process of it, but I know accuracy potential forever disappears once a bore swages bullets down at an early point in their travel. Even further barrel setbacks won't help once this happens.
Officially dead..

Now, what I 'think', is that carbon impinges itself into the bore(somehow), lifting it's surface. And I don't think this can be reached with abrasives -without damaging the bore as badly.
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Old 03-02-2011, 12:34 PM
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Re: Barrel Wear

Kevin & Fitch great post-I needed to know that.

Please go into more on the single based propellant vs double based powder - I know nothing.
Like, what you get with each one ie. every thing brings BAGGAGE to the party- what does each bring when you use it.

I have long believed that erosion all starts with the primer chemicals selected by the mfg. Company, especially the makeup of the clear-coat-or whatever- sprayed on to hold things together. Don't remember much from Chem class (100 yrs ago-lol) but some perfectly safe and useful stuff, when the environment is changed to the extreme (TEMP.), can go BALLISTIC-pun intended! I use the hot GM215M for speed in my one load for my 338WM. Please share any info you have gained on said primer chems and or COOLER firing ones that may be used when not needing the hottest loads. Should help save on erosion.
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