Throat Erosion and Barrel Life

Marksmen

Marksmen

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I putting this out there to generate a consensus among experienced shooter's and gunsmiths.

Throat erosion is a fact of shooting every round wears out the barrel. Barrel life is based on the stress in the throat area?

The typical center-fire cartridge erodes at approx .001" per 100 rounds?

A typical factory barrel on a 308 throat erosion is .035" after a 1000 rounds?

6.5X284 custom rifle as much as .066 in 500 rounds?

Designing a chamber reamer around a particular bullet (lead in angles and such) be of a benefit if any at all for some 200 rounds?

I'm not talking about a chamber reamer to suit factory ammo like Federal match which is more to do with seating depth.

I am looking forward to peoples thoughts on this subject.
 
i think you have pretty much nailed it and if you looked at the throat with a borescope to decide when a barrel was done, you'd be throwing away some damned fine shooting barrels. judge by how well they perform, not the looks. much like women i guess.

there is a formula floating around that is based on a case capacity in grains of h20 and dividing the frontal area of the case mouth in square inches. the higher number the harder it is on barrels. so, something like a 30 whisper will almost never wear out and something like a 22-284 would toast in only a couple hundred rounds.

the combination of intense pressure and powder ignition that generates enough heat to vaporize steel and that heat is concentrated at the throat. i think the hotter the steel is at the time the round goes off, the more throat that vaporizes so if time allows shoot slowly for best barrel life.

hunting rifles should last forever regardless of cartridge because how many times are they shot in a lifetime if used for hunting only. they have pencil thin barrels to make them light and they heat up very fast and change dimensionally so using a hunting rifle for target shooting is going to wear it out much faster.

as to lead in angles i cannot see how that will wear a throat any more or less but some lap jobs are so bad on some barrels that they are choked at both ends and it is a ##### because you either cut the shank end off or you ream with a bushing that isnt properly sized
 
paperslayer said:
i think you have pretty much nailed it and if you looked at the throat with a borescope to decide when a barrel was done, you'd be throwing away some damned fine shooting barrels. judge by how well they perform, not the looks. much like women i guess.

there is a formula floating around that is based on a case capacity in grains of h20 and dividing the frontal area of the case mouth in square inches. the higher number the harder it is on barrels. so, something like a 30 whisper will almost never wear out and something like a 22-284 would toast in only a couple hundred rounds.

the combination of intense pressure and powder ignition that generates enough heat to vaporize steel and that heat is concentrated at the throat. i think the hotter the steel is at the time the round goes off, the more throat that vaporizes so if time allows shoot slowly for best barrel life.

hunting rifles should last forever regardless of cartridge because how many times are they shot in a lifetime if used for hunting only. they have pencil thin barrels to make them light and they heat up very fast and change dimensionally so using a hunting rifle for target shooting is going to wear it out much faster.

as to lead in angles i cannot see how that will wear a throat any more or less but some lap jobs are so bad on some barrels that they are choked at both ends and it is a ##### because you either cut the shank end off or you ream with a bushing that isnt properly sized
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question
how long (rounds) do you thing a perfectly matched bullet-throat combination would last on a brand new chamber before all that is washed away down the barrel?
 
My observations indicate the following:

visual bore inspection does little to tell you what the actual accuracy will be. Only holes in paper matter and the further away you test, the better.

The lose of rifling in the throat is the most obvious way a barrel looses accuracy. But that lose can extend much further then some suspect. how about 4" down the bore?

Bore wear is non linear. I feel that the steel stays strong for most of its useful life then degrades rapidly through to the end. This can happen in as little as 1 relay - been there, done that.

Types and chemistry of powders, pressures/heat and how hot the barrel was in use cause bore life to massively swing above or below the norms

In general, the throat angle has little to no affect on accuracy as it is changed so rapidly under use.

The beginning of the rifling does play a sig role on OAL set up. Ideally, I set up my OAL for best bullet seating/case capacity leaving the ogive a few thou OFF the lands. The relationship will change but usually stays put during the bulk of the useful life.

Being able to engrave the lands DOES NOT MEAN THERE IS ANY RIFLE LEFT BEYOND THAT POINT. You can have a lump of rifling left while the rest is gone just millimeters beyond.

There are more variables then the above so I have given up trying to quantify bore life. Keep good notes and track how the barrel SHOULD behave. When things dont follow trend and shot count gets over 1000/2000, best to have another barrel ready to drop in.

We all know that the 308 is a very long lasting option, however, some enterprising FTR shooters have been able to hot rod the 308 to significant increases in performance. The downside, these barrels have cooked in as little as 1000rds.

There is no free lunch....

Jerry
 
Jerry, whats your opinion on moly coating to improve barrel useful life? Norma ammo has a good writeup online about their own in house testing from which they claim moly increases barrel life fractionally, which appears buyable evidence; but then I also think I remember reading a story of moly ruined barrels, something about a super tough residue ring which may build up near the throat with extanded moly use.
 
rimfiremac said:
Jerry, whats your opinion on moly coating to improve barrel useful life? Norma ammo has a good writeup online about their own in house testing from which they claim moly increases barrel life fractionally, which appears buyable evidence; but then I also think I remember reading a story of moly ruined barrels, something about a super tough residue ring which may build up near the throat with extanded moly use.
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Moly is a high pressure lubricant.

Beyond that, there is no scientific evidence to back up any product claims. at least none that I have seen.

Can it help prolong shooting before fouling becomes an issue, yes.

can it help with a rough bore maintain accuracy and velocities, yes,

Does it help with bore life? no idea as I have never done any direct testing to prove this either way. And I have not seen any tests that proved this outcome beyond anecdotal.

Some swear by it, some swear at it. I used alot of it over the years but don't anymore.

Might look at it again but more from the standpoint of lengthening between cleaning cycles.

But then it could rain....

Jerry
 
Stainless barrels erode in the throat differently than a blue barrel.

A blue one loses metal and the rifling gets washed away.


Stainless throats look like they have more rifling, but the throat developed cracks, looking like dried mud. Eventually a piece of steel breaks off between a set of cracks and the accuracy is gone in that one shot.

Stainless barrels tend to last longer with less gradual decay in accuracy.
 
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I had a kreiger 6.5 barrel chambered in 260 (custom tactical rifle ) I had set back at 2200 rnds... I shot it till last Monday and it still cloverleafed a 3 shot group and shot very accurate out to 600 meters.. with a total round count of 4300 I'm surprized it shot at all.. Mr. Rempel bore scoped it when he took it off and was also surprized it still shot.. just one of those barrels I guess...
 
This always interested me but I've never heard any first hand stories.

[youtube]jiegZyhd5l0[/youtube]
 
KDX said:
This always interested me but I've never heard any first hand stories.

[youtube]jiegZyhd5l0[/youtube]
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I call BS...

How does a surface coating effect a meaningfull change in the base metal aside from lubrication?

With stainless and cromo barrels you have several things all working against you... Gassing off of base metal with heat from the propellant, tempering of the base metal from the heat, pressure expanding your chamber and throat acting on the tempered surface area as the shockwave passes thru to a maluable area forcing pressure cracks to develop in the hardend surface material (see "dried mud")... And that's without even pointing a finger at lost elasticity of the chamber area as a whole from 50,000psi cycles constantly expanding and contracting the walls.

.....remember "dura lube".

Lol....
 
Man, I'd be re-barreling at least one gun every year. I've put over 2500 thru my 308 since I got it last may, and that's the gun I can afford to shoot the least.

I'm sure many members here could say the same.
 
Interesting I have salt bath nitrated the steel rings I used to make. It made them impervious to rust and made the surface of the metal incredibly hard. The process reacts to the carbon in the steel and turns it into a nitrate. Is is what pretty much the standard now with machine tools, nitrating greatly improves tool life.

Nitrating or anodizing are not coatings. They change the metal to a different compound.
 
Bath Nitriding is definitely something I want to explore. Got to get through my present work load to follow it up.

There is no doubt, the process does work in industrial settings. The steel surface is dramatically harder. I have chatted with enough machine shop and engineers who swear by it for their applications.

The only question is does it change the function of the bore and its affect on accuracy?

Not many want to risk their good shooting barrel to a several hundred dollar experiment.

Jerry
 
I'd imagine there should be done imperical internet evidence if thr effectiveness if notifying from folks stateside... Maybe on the good old SH? Those folks seem pretty serious about their toys.
 
AR-15 barrels and parts are starting to go the way of "Nitriding". Perhaps cause it's easier then crome lining which I head does not lend well to accuracy.

NEA ARC+ treatment is a form of Nitriding from what I understand.
 
the combination of intense pressure and powder ignition that generates enough heat to vaporize steel and that heat is concentrated at the throat. i think the hotter the steel is at the time the round goes off, the more throat that vaporizes so if time allows shoot slowly for best barrel life.
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This is the mechanism of throat erosion, it is entirly due to heat/temperature attained by the surface atoms of the barrel steel. The flame temperature of smokless propellants is in the order of 2500 C; the melting point of AISI 4140 barrel steels is approx 1500 C. The throat area does NOT melt away due to sufficient barrel mass to conduct enough heat away from this area to prevent melting. However there is enough heat energy available in the throat area to vaporize the surface atoms of the steel. This process is called sublimation. Pressure does not contribute to the erosion process, in fact higher pressures favor the solid state, not the vapor state. Nor does bullet friction contribute to erosion, bullet velocity is lowest in the throat area and highest at the muzzel.
 
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...so, if I understand that process correctly, which I think I can wrap my head around as plausible, the sublimated particles leave the barrel behind the bullet with each shot. This explination on wear logically describes why higher power cartridges have shorter useful lifetimes, as the greater volume of powder promotes greater high temperature linger times in the throat, which in turn sublimates a greater number of molecules with each shot. I would suspect powder volume to bore size would play into the equitation on the basis of surface area alone, explaining why a 260 Rem has a shorter barrel life than a 308 Win.
 
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