Throat Erosion and Barrel Life

thecollector said:
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.
Click to expand...

Pressure does indeed contribute to throat erosion, as it is directly correlated to chamber temperature ;)
 
I would challenge the statement that Stainless barrels have any appreciable advantage when it comes to throat erosion.
Most barrel grade stainless steel is alloyed so it is more machinable. This usually involves the addition of sulphur.
Since sulphur will burn out of the alloy, it leaves an eroded surface in the stainless that cancels any advantage that stainless might otherwise have.
I have not noted any measurable barrel life advantage in stainless steel barrels.
Eagleye.
 
Jordan Smith said:
Pressure does indeed contribute to throat erosion, as it is directly correlated to chamber temperature ;)
Click to expand...
So following the above theory, how would that explain that while the 308 Win has a max SAAMI recommended pressure of 62,000 psi, and the 260 Rem is max 60,000 psi, the former has a history of typically having a much longer barrel life than the later?
 
thecollector's description of wear might also give insight into how moly may extend barrel life marginally, not by providing lubrication alone, but by surface coating the bore with a material that has a melting point approaching 2700C, thus reducing the sublimation of the bore steel with each shot
 
The physics of the pressure theory just doesn't seem to hold logic. Few materials erode or otherwise evaporate under high pressure alone. Pressure could be coupled with velocity, but not near the throat, where both bullet and gas velocity should be low.
 
Barrels are a consumable just like bullets and powder. When the throat erodes you really have 2 options. 1. cut the threads off, rethread, rechamber and carry on for maybe another 1000 rounds or so or 2. Just rebarrel and get it over with.

Well I guess there is a 3rd option... Just post it on the EE as a tack driver with a low or unknown round count, that was only shot on Sundays by a little old lady:p

I have seen .308 Target Rifles with over 8000 down the tube, the shooter still made the Bisley Team with it.

Another friend had a .223 target rifle that his gunsmith said "there was no way it could still be accurate. No rifling for the first 6 inches", still won a club championship with it.
 
rimfiremac said:
The physics of the pressure theory just doesn't seem to hold logic. Few materials erode or otherwise evaporate under high pressure alone. Pressure could be coupled with velocity, but not near the throat, where both bullet and gas velocity should be low.
Click to expand...

Do a little reading about the relationship between pressure and temperature, and you'll see how pressure affects throat wear.
 
Originally Posted by Jordan Smith
Pressure does indeed contribute to throat erosion, as it is directly correlated to chamber temperature
Click to expand...

At the temperatures of a cartridge discharge (some 2500 C) the ideal gas law is fully applicable:
P=(nRT)/V most assuredly.
Pressure correlates directly with absolute temperature, this is only applicable to the gas phase and merley characterizes how the variables of P,T,V,n interact with each other and does not describe the solid phase of the barrel steel, nor the dynamics occuring at its surface.
Erosion appears to be greatest in rounds exhibiting the highest pressure as these rounds achieve these high pressures by virtue of the fact that they contain the largest powder charges (by weight relative to bore area), hence they have the greatest enthalpy (heat) content and therefore the greatest erosive effect on the throat area.
 
Last edited:
Missing one very important dimension - TIME.

The most erosive cartridges burn the most fuel for a given bore size at the max allowable pressure - the energy is delivered to the steel for a longer period of time allowing the steel to reach a higher peak temperature and for a longer exposure.

More energy in - hopefully - more work done.

A cartridge buring 80gr of powder at 65000psi will be more erosive then a smaller cartridge buring 40gr of powder at 65000psi - all else being equal.

But then we could also throw in variables like single and double base powders, the actual alloy of steel in any particular barrel, the depth of rifling and more anecdotal traits like long case necks, shoulder angles and shapes, phase of the moons and box office sales of summer movie blockbusters.

As they say YMMV....

Just go shoot the rifle and when it doesn't do what you want or expect, put on a new pipe.

Gave up asking why many barrels ago....

Jerry
 
thecollector said:
At the temperatures of a cartridge discharge (some 2500 C) the ideal gas law is fully applicable:
P=(nRT)/V most assuredly.
Pressure correlates directly with absolute temperature, this is only applicable to the gas phase and merley characterizes how the variables of P,T,V,n interact with each other and does not describe the solid phase of the barrel steel, nor the dynamics occuring at its surface.
Erosion appears to be greatest in rounds exhibiting the highest pressure as these rounds achieve these high pressures by virtue of the fact that they contain the largest powder charges (by weight relative to bore area), hence they have the greatest enthalpy (heat) content and therefore the greatest erosive effect on the throat area.
Click to expand...

As you said, the ideal gas law is PV=nRT, or P=nRT/V. This isn't totally representative of the gas molecules in deflagrating powder, however, because pressure is so high that the gas behaviour begins to move away from that of ideal gas. Although you're right that at those temperatures the gases will tend to approximate ideal gas characteristics. So while not a perfect representation, the ideal gas laws would certainly apply here.

I completely agree with your description of the chemistry and physics at play here, as well as your reasoning. The only thing you're forgetting is that the expanding gases are what transfer heat energy to the steel at the throat, and they most assuredly do increase in temperature as pressure goes up.
 
Mystic Precision said:
Missing one very important dimension - TIME.

The most erosive cartridges burn the most fuel for a given bore size at the max allowable pressure - the energy is delivered to the steel for a longer period of time allowing the steel to reach a higher peak temperature and for a longer exposure.

More energy in - hopefully - more work done.

A cartridge buring 80gr of powder at 65000psi will be more erosive then a smaller cartridge buring 40gr of powder at 65000psi - all else being equal.
Click to expand...

Yep, that too. A longer blast of flame at the throat has an exponential effect on erosion.
 
i would be very interested to see a thorough experiment done by barrel manufacturers every few years, take a few of each barrel they make (stainless, regular, different rifling profiles of each and any bore coatings or treatments) and fire a selection of ammo through them. try moly and non moly, boat tail and flat base, cast and jacketed, and popular powders behind each. does a flat base moly coated 150g .308 going 2700 fps with varget wear out a stainless barrel faster than a non coated 150g BT going 2700 fps with 3031 wear out a regular barrel? any info we currently have is little more than SWAGs since noone has done any real experiments.
 
And why would any barrel maker open this can of worms?

The only thing ANY match barrel maker cares about is the first 200rds down the pipe. By then, the customer is either thrilled and off to win the world championships or they have to deal with warranty.

Beyond that, what incentive do they have to worry about barrel wear under uncontrolled circumstances?

heaven help the barrel maker that offers any claim of barrel life AND accuracy.

If you want long barrel life, talk to the manfs that cater to the men and women wearing uniforms.

But accuracy will likely not be stellar.

Jerry
 
Do you guys subscribe to the theory that certain shoulder angles cause the ignition flame to reach an apex further down the barrel thus causing less wear at the throat?
 
You must log in or register to reply here.
Back
Top Bottom