Shooting a hot barrel?

[JayBeeYou]

Honest question that some of the smart guys on here can answer. I know convential wisdom is that shooting a barrel when hot greatly increases the rate of throat erosion. I do not doubt this and as a FTR shooter I accept there are times I will do all I can to torch a barrel in a calm or steady condition. On a really hot day it is not uncommon to get a barrel north of 200 degrees.

My question is why this 200 degree headstart causes a increase in the wear on the barrels throat when the heat potential starts at a low of 3720 degrees for VV N140 to a high of 4050 degrees for Varget among the typical FTR powders. I assumes that amount of heat generated during firing is actually higher than those numbers as we are inducing high pressures during the actual powder burn. In my mind this extra 200 ish degrees is within the variance of different powders and the phrase match on a fire comes to mind.

Not going to try and convince anyone that you shouldn't worry about getting barrels hot, just looking for an explanation of what is the mechanism of increaed barrel wear.

 

[Bill K]

My thought is, first that 200 degree temp you register is on the outside/top of the barrel, let's say over the chamber area, but the temp inside at the throat is way higher and the friction there too boot, is what will cause the wearing away of the metal. So when it is much higher than you read outside along with the friction of the bullet on the metal, that is what causes the wear. You sit there and fire a hundred or so fast shots with those high temps at the throat, you will wear that area out pretty darn fast.

 

[jelenko]

With the thermal conductivity of stainless steel, the temperature after/before a shot is not much higher than 200. Think of the a one inch sheet of stainless steel - heat one side to, say, 200 degrees - the other side is going to get pretty close to that.

That said, what is taking material off inside the barrel is the hot gasses from the burning powder - essentially vaporizing some of the metal. My understanding is these gasses are several thousand degrees F. Still, as the temperature of the barrel increases by even a couple hundred, the amount of vaporized material will increase.

 

[rwj]

It is well known that when heat is applied to a steel part, it’s volume increases due to thermal expansion. This is also true for a hollow cylindrical shape, but a somewhat unexpected thing occurs when heated is applied only to the internal surface, it initially expands radially inward - its ID becomes smaller. Not until the temperature begins to soak thru the entire cross-section does the ID begin to get larger.

Apply this to a bull barrel that is briefly exposed to high temperatures primarily near the throat, this area becomes smaller in diameter after a few shots. This increases frictional forces on the bullet and related erosion. How much is dependent on many factors...

 

[BCBRAD]

It is well known that when heat is applied to a steel part, it’s volume increases due to thermal expansion. This is also true for a hollow cylindrical shape, but a somewhat unexpected thing occurs when heated is applied only to the internal surface, it initially expands radially inward - its ID becomes smaller. Not until the temperature begins to soak thru the entire cross-section does the ID begin to get larger.

Apply this to a bull barrel that is briefly exposed to high temperatures primarily near the throat, this area becomes smaller in diameter after a few shots. This increases frictional forces on the bullet and related erosion. How much is dependent on many factors...

Any one who has used heat to have an interference fit on a coupling to shaft knows this. Also, as the heat is applied the coupling /rifle barrel will 'close in' this can leave the hole oblong/not perfectly round to an extent, heat soaking for a minute (short period of time) will give the hole time to distribute the heat (soaking) thus making the hole round again. The gas generated in the combustion event has particles of powder entrained in it, this is abrasive and will erode the throat area first then over time down the barrel.

 

[rwj]

Any one who has used heat to have an interference fit on a coupling to shaft knows this. Also, as the heat is applied the coupling /rifle barrel will 'close in' this can leave the hole oblong/not perfectly round to an extent, heat soaking for a minute (short period of time) will give the hole time to distribute the heat (soaking) thus making the hole round again. The gas generated in the combustion event has particles of powder entrained in it, this is abrasive and will erode the throat area first then over time down the barrel.

What percentage of the population do you think has such experience?

 

[BCBRAD]

What percentage of the population do you think has such experience?

Any one who has taken at least high school physics.

 

[Straightshooter1]

With the thermal conductivity of stainless steel, the temperature after/before a shot is not much higher than 200. Think of the a one inch sheet of stainless steel - heat one side to, say, 200 degrees - the other side is going to get pretty close to that.

That said, what is taking material off inside the barrel is the hot gasses from the burning powder - essentially vaporizing some of the metal. My understanding is these gasses are several thousand degrees F. Still, as the temperature of the barrel increases by even a couple hundred, the amount of vaporized material will increase.

Yeah, I think you're pretty much right on about the hot gasses as it relates to barrel wear. And I'd add that with that and a combination of other factors add to it . . . like friction and the expansion and contraction of the barrel as it heats up and cools down. The firecracking we observe in our barrel is visual proof of the degradation of the metal and seems to me easy enough to understand how a material the has that kind of cracking can loose material from those forces. From all the studies I've read, it seems the thing that is the primary cause are the hot propellant gasses.

My understanding is this . . .when a barrel heats up (like to 200°F), it expands. When it expands the diameter of the chamber and bore become larger. This expansion results in the bullet not fitting as tight as it would when at the cooler temps and therefore why we will see degradation in accuracy as barrels heat up (pencil barrels heating up much faster than bull barrel, so it's seen much sooner). But, as far as wear goes, the greater the expansion and contraction the more tress is put on the material and accelerates wear in the area of the lands where the hot gasses do the most damage will.

For some of the science behind some of my thinking, here:

Study on performance degradation and failure analysis of machine gun barrel

An increase in the use of the gun barrel will cause wear of the inner wall, which reduces the muzzle velocity and the spin rate of the projectile. The…

www.sciencedirect.com

. . . and the attached.

 

[B Nettesheim]

I usually don’t shoot out barrels but when I do I keep them hot

 

[rwj]

Any one who has taken at least high school physics.

Doubtful based on the number of people I’ve had to educate that passed college level physics. Theory and application are very different!

 

[K22]

I avoid rapid fire sequences when ever possible. For me, this is not difficult because I'm primarily a hunter even though I spend a fair amount of time at the range (2 to 3 days a week) in practical practice sessions.

The only time I find it necessary to shoot out of a "hot" barrel is for follow up shots while hunting which isn't that often and even then it's only 1 or 2 shots at the most. At the range I normally only shoot 10 to 15 rounds per range session which typically takes me about an hour and half or so. The goal here is to stay current and sharp on the fundamentals so when that monster ground hog or predator shows itself I'm in the zone.

My approach is more focused on performance than barrel wear. A friend of mine, a former State Police sniper influenced my approach. His mantra was - it's the cold barrel shot that was the most important. In hunting, most of the time it's the cold barrel shot that matters the most. So my practice routine is focused on trying to duplicate a cold barrel shot with each shot during range practice.

However it kind of makes sense that shooting rapid fire would accelerate barrel wear - I wonder how long the barrels on those AR's last when I see guys at the range shooting 10 to 20 round strings in rapid fire. Oh well, each to his own.