Actual couse of barrel wear?

[Lemonaid]

I saw this video and thought I would pass it on for discussion. Main theory is that it is fire cracking is caused by pressure expanding and contracting the barrel, not heat or friction except as a lesser contributors.
There are some good points made in the comments section so check them out as well.

https://www.youtube.com/watch?v=BhBrCU_M3fM

Dang it, how do you edit a typo in the thread heading?

[grayfox]

Got 6 mins into it and quit, frankly - boring. No facts or figures. "A lot..." is, well, how much?
And to be precise, he's not talking about barrel wear per se (ie, wear down the bore of lands/grooves) but throat erosion.
Last I checked, isn't the throat larger diameter than the bullet? So bullet friction won't affect the throat much.

Not sure where he's going.
One more thing, sounds like he's putting his one "shot-out" barrel against hundreds, maybe thousands, of competitor barrels that folks have measured jump-movement, ie, throat erosion, from start to finish of a match.

I'd much rather hear from some materials science guys or specialized engineers who have some real info to share.

My $0.02 anyway.

Maybe I missed what he's meaning to say big time, but that's my take.

[Lemonaid]

[grayfox]

Sorry last night I was probably a little cranky and tired...
Here's what I think I know on the subject, we did deal a lot with metals and steels and such in the day.
The powder does not explode in the chamber so that's why the steel doesn't blow apart... it is a subsonic flame-wave propagation which is good b/c blowing up the steel is basically a brittle-failure type of action by the steel. So the steel can stay within its ductile properties.
Some gasses escape down the barrel before the bullet seals that pathway off, so that gas/particulate puff can do some small amount of erosion/ablation in the throat area right at the very start of the shot.
Then the bullet seals off that pathway, so at that time during pressure buildup gas/particulate velocity is limited by the speed of the bullet itself. When the bullet hits the muzzle powder and particulate can escape and they do, at ~3x the speed of the bullet, so back at the throat again erosion/ablation can occur, now at an obviously high temperature (I'll look up that typical temp in a minute...)
Ok, I'm back...
found one military study online, from 2013, a590866-thermo-of-rifles.pdf, (the thermo of rifles is title I added, google the a59086 number)
and a study of barrel temperature for an olympic rifle shooting style scenario, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3944560/. 'Course that's probably a 22 or 223 small-case round, but still.
Most treatments of energy transfer work from an assumption that the PVT behavior in the chamber/barrel is adiabatic, ie, not much heat transferred to the barrel.
There is also, however, this page talking about Naval guns and internal temperatures:

https://eugeneleeslover.com/USN-GUNS-AND-RANGE-TABLES/TEMPERATURE-OF-THE-GUN-AFTER-REPEATED-FIRING.html

I didn't exactly find the actual chamber temperature however.

As to the stresses by the temperature delta, our nuclear industry also would use a 2-layer approach to steel and thermal stresses for some important, thick steel components. The stress placed upon a layer of the steel due to temperature delta is opposite to the temperature gradient. IE, when the inner surface temp is hotter than outer (during the shot), the stress is compressive on the inside layer and expansive on the outer. During the cooldown after the shot, when the outer temperature becomes greater than the inner, then there is a tensile/expansive stress (that would tend to open up a crack or flaw) on the inner surface. Meaning that "cracks" from heat stress probably don't occur from the inside-out heatup during a shot.
Surface hardening can occur.
Erosion/ ablation can occur.
If there is a pre-existing flaw (crack) then the erosion can enlarge it.
I'm not convinced that the heat and flame causes the crack during the shot however.... but I'm not an expert either.

Hope this makes some sense and gives some room for thought and discussion.

[grayfox]

Page 29 of that Army research paper (a59086.pdf) seems to say that at the point of complete powder burn in an "ideal gun" (where the bullet is somewhere down the rifle barrel but still in the muzzle) the max temperature of the gases/system is 2884 Kelvin, or ~4700 deg F.
That's pretty hot.

[grayfox]

here's another article from the PRS blog, it's a good one I think.

https://precisionrifleblog.com/2020/03/24/how-fast-does-a-barrel-wear/

For several match grade barrels and calibers he measures throat change/ erosion per 100 rounds, from about 0.004"/100 to 0.077"/100.
Plus some other good links on the topic.
I like what these guys write b/c they are in real world competitions and it's where the really good shooters are.
From the start to the end of a given match their barrels can change enough that they see or adjust for accuracy deltas to stay in the running.

[Lemonaid]

The theories put forth in the video dove tail into another relevant topic, do some powders erode the barrel faster than others? I've been waiting for someone to do a test to see which are the mildest and which are the harshest.

[grayfox]

What I was reading there says powder choice, cartridge choice, over-bore tendencies, and powder charges all can factor in.

[Mad Charlie]

Since rifles that are never fired never wear out, it stands to reason that shooting causes barrel wear.

[ricsmall]

Since rifles that are never fired never wear out, it stands to reason that shooting causes barrel wear.

^^^+1^^^. Lol!! So true

[Bobke]

From a chart extracted from Quickload, the following may provide some powder selection criteria that might mitigate throat erosion to a degree. Is a powder flame temp rating, not a burn rate chart, for powders we commonly use in Grendel. Glad to see AR Comp at the low end of range.
N140: 3720
AR Comp: 3740
N130: 3770
748: 3840
Benchmark: 3900
CFE223: 3920
XBR 8208: 3920
TAC: 3950
H335: 3980
BLC2: 3990
H322: 4000
H4895: 4060

[Lemonaid]

Thanks Bobke, good info to know. If you have access to Quickload could you run a few average loads (not max, not min) and see what powders give the longest time/length in the barrel before peak pressure is reached? I'm thinking there may be a relationship between the length before peak is achieved and erosion rate.

[Bobke]

[Lemonaid]

Does the burn rate of a powder translate to time to peak pressure? Fast burning powders peak faster than slow burning powders? Seems like it should but want to fact check my thinking.

[Bobke]

Would think your logic would be accurate.