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nemesisenforcer Member Posts: 10,513 ✭✭✭
edited January 2014 in Perry Shooter Competition Shooting and Reloading Forum
is most barrel wear caused by gas cutting or jacketed bullet on bore friction?
"is most barrel wear caused by gas cutting ".........for sure, bullets (copper or lead) cause VERY little wear. the old 'rule of thumb' is a barrel will last for 40 lbs. of powder.
Hadn't heard that, but it makes some sense. I'll keep that in mind.
sloppy cleaning from the muzzle end can cause degraded accuracy, but can be cleaned up by trimming and re-crowning.
quote:Originally posted by JustC
most "burned out" barrels suffer from throat erosion, not from rifling wear. The rifling wear would be worst at the throat because that's where the bullet is being engraved. Heat from the powder and from the friction of bullets already fired would make the metal at the origin of the lands more wear prone.
The erosion is the worst at the throat because that is where you are basically applying a plasma torch to the rifling every time you touch off a round, moreso if you are running "hot" home-rolled loads.........As usual, JustC hit the nail on the head
High-nitroglycerine double base powders like Cordite and Hi-Vel No 2 are considered to be more erosive than straight nitrocellulose.
Bruce Hodgdon always maintained that Ball powders were less erosive than extruded, but nobody will back him up.
quote:Originally posted by jonk
One country, I think Switzerland, did an experiment, where they fitted a rifle barrel to a special compressed air tank that produced the same pressure as one of their service rounds. They fired a ton of projectiles through it, and reached the conclusion that there was virtually no wear, ergo all wear must come from the gas cutting.
This is not the experts forum so I am entitled to my guesswork.
Five factors are not present with compressed air.
The heat of firing may anneal the very surface of the origin of the rifling, leading to reduced wear resistance. The air test would not show this unless the barrel were heated between shots with a non-erosive method such as a laser beam which affected the very surface of the barrel, right where it gets hottest in real firing.
Heat itself can affect the amount the barrel wears even if the very surface had never been exposed to temperatures which could anneal it slightly. Compressed air expanding would keep the barrel cool.
In the real world, bullets may have grit on them.
Copper fouling may have protected the origin of the lands, firing would have shot it off, if it can erode the very steel.
The lube situation may be different. Bullets may have had a remnant of forming lube, which firing may blast off as hot gases race around the bullet. The gas flow may be too fast to allow it to be replaced with graphite.
What this adds up to is that a different bullet may wear the throat less. A bullet with a shorter shank or thinner jacket may wear the POSSIBLY heat softened metal less. It would create less frictional heating and maybe leave the metal more resistant to gas erosion. Grit would not be as bad with a short shank. And, by sealing the bore sooner or not lingering in the throat so long, with hot gases racing around an imperfect seal, it may reduce how hot the throat steel is before it has to take the full blast of the powder.
Even if your hypothesis is true, it reinforces, not undermines the hot gas at the throat analysis above.
IF, annealing the barrel throat were to ever happen,..the throat in the barrel would be eroded in 200-300rnds, and the barrel would need to be replaced. Annealing takes place at some 600-700* on brass,....steel is a whole nother story.
Bullets with "grit" on them are either dropped in the dirt, and not cleaned off before chambering, or those garbage "fire lapping" rounds sold by folks who play on the lack of internal balistic knowledge of most folks.
here is one of a few sights I found courtesy of google, that discusses annealing steel.
judging by the description of using a fire-box and excluding air from the process, I can't see how even a long shot string of overbore magnums could create enough temperature, throughout the barrel's thickness, while not having the air inside the bore, negate the heating process inside the bore at the throat in front of the chamber. The exiting plasma is dissipated in milliseconds as it is constantly expanding and becoming less dense, as the projectile is moving toward the muzzle, and thus is increasing the internal volume of the compression chamber which is the barrel chamber and bore behind that projectile.
an annealed metal doesn't resemble alligator skin,..eroded and fire-cracked steel does.
every "worn" barrel I have ever seen is from being shot-out at the throat, which by definition, means they are always fire-cracked. The rule of thumb is that 40lbs of poweder, burns a barrel out (or to be correct, erodes enough of the rifling lands away, starting at the throat and moving forward, to degrade accuracy) Lets say a 308 using 50grs of powder is the test rifle, which obviously would get 140rnds out of a pound of powder, and therefore would be "worn" out at 5600rnds. IMHO, if you were softening that steel to any real degree during each firing, I don't see how the rifling would last 5600rnds. However, as the distance from the case mouth is increased, the erosion becomes less and less as the plasma has become less dense and has become more of a fire as it extends down the bore. The rifling at 5+ inches from the case mouth, is usually almost like new, with respect to it's height, with some wear at the corners where the bullets engrave their jackets. That alone proves that the vast majority of erosion, is caused by exposure to the plasma that exits the case mouth, but only last tiny fractions of a second, thus the amount of firings needed to erode the throat/rifling. This is not an ar15 or ak etc that is simply run hot at the range while the owner gets the feeling of dumping 30rnd mags into a target 25yds away, of course.
99+% of hunters, will never wear a barrel enough to even notice a change in the throat area. They simply don't shoot them enough times, or run them hot enough to burn them. Most rifles, even a pre-64 Winchester, have only been fired a few hundred times in their life. This can be proven by running a bore scope in the barrels of trade in guns at the gun store, a practice that most owners employ, since they don't want to have to eat the cost of a rifle they took on trade. have taken bore-scopes to the store to inspect the throats of rifles I was interested in. If the shop owner didn't like it, then I would have to question why.
that's my take on barrel "wear" (erosion) YMMV
Most of the throat is worn away. While a couple inches down the barrel all the way to the muzzle is near perfect. A bullet sliding 20-25+ inches down the barrel not causing much if any wear, losing significantly less than the throat, which absorbs 90+% of the hottest gasses.
That tells you right there, high pressure gasses cause the most erosion.
My age old analogy applies directly to this topic as well.
"Think of a round going off in the barrel like tuning a blowtorch for cutting. When you have O2 and acetylene just burning you see a long mild blue tip. Turn the mix just right and you see a sharp bright blue tip. That's where the metal gets cut."
Soreshoulder brings up a good point. But, as nemesis said it reinforces not undermines the subject.