How can throat wear be predicted?

M

Mirage416

New member
Obviously having a long bullet which is pushed into the throat very fast is what constitutes a barrel burning cartridge. It seems this equates to a high BC bullet leaving at say 3000FPS+ (as an a example).

Here is what confuses me.

Now if we have the same length of bullet and the bore becomes larger, thus BC goes down, but velocity still remains the same, it sounds from reading that this becomes less of a barrel burner. Why?

To me it seems that the primary factor should be length of copper which is engraved each time it passes the throat, regardless of diameter of the bore. I know there are more minute factors like jacket/barrel hardness and etc.

So can anyone explain better what actually predicts just how destructive a certain cartridge will be to the throat?
 
From what I`ve been led to believe it is a combination of heat and pressure that destroys a throat not the bullet passage. Wear is a small factore where the real damage is from high pressure gas erosion.
 
I did not even think of that. That would explain why a larger caliber with similar bullet would erode less, because it is like a bigger nozzle on a rocket for gasses to escape, all else being equal. Interesting.
 
How to predict throat wear

Think of throat erosion as being proportional to the ratio between case volume and bore diameter. For a parent case, the smallest caliber of bullet will produce the fastest rate of erosion. A 6mm-06 will suffer faster throat erosion than a 25-06 which will be worse than a .270 and so on. The largest case for a given bore size will be the worst for throat erosion.

There have been a truckload of theories advanced to complicate this, and some of them may have merit. Some believe that heavier bullet loads erode faster than light bullet loads in the same cartridge. Most of the reloading manuals caution against shooting more than three shot groups with hot magnum rifles on the premise that the throat erodes faster if the barrel is allowed to get hot. There have been claims made that the length of the cartridge neck and the shoulder angle influence erosion, but recently that premise has come under criticism. I won't even go near the claims that lubricants such as moly etc. reduce erosion.

We know for sure that smaller, more efficient cases produce less erosion. This is convenient for those of us with the short range benchrest habit!

Scott Roeder
 
If steel is eroded by the gas which is forced to converge upon the throat, how much is the softer brass in the cases eroded in the neck area upon each firing? Has anyone found brass to erode or thin more than they desire in the amount of firings the brass is typically good for? Or maybe this itself is what actually constitutes the life of the brass. :confused:

I also picture powder granules having an influence on throat erosion to some degree, now that the idea is better grasped. I mean, if you have solid particles flowing over steel at high pressure and velocity, those would in theory erode the metal more quickly than a gas.
 
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Predict throat erosion ??? Why ??? Measure it ... every 300 to 400 rounds instead.
 
abintx said:
Predict throat erosion ??? Why ??? Measure it ... every 300 to 400 rounds instead.
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I didn't mean it in the literal sense of predicting an exact number. What I was getting at was just how to achieve a general estimation for comparison of similar cartridges.

It was one of the things I never understood exactly, and wanted to clarify.
 
SGS has it nailed IMO.

And to answer your next post mirage, the process involved is most often termed "ablation" as opposed to "abrasion." The actual abrasion by the powder granules isn't a large factor.

Ablation is the gasifying effect of the high temperature gas impinging on the surface of the steel and actually carrying it away.... "melting" it. This coupled with the "firecracking" or differential shrinkage cracking induced by the temperature differential between the surface and substrate of the steel is what wears out the throat. Kinda' like you can run your fingers through the flame of a butane lighter but don't try it with a blowtorch or an oxy/actylene jet! And, once the surface is roughened by the shrinkage cracking (think dried playa lake bed like the Bonneville Salt Flats) then the increased surface area and fragile edges are more easily "melted" and carried away by the gas jet. AND, larger cases have more excess hot gas being used just to melt the steel.


Another little explored factor is gas leakage around the bullet. Generally the large overbore cases use slower powder which means that the bullet obturates, upsets or seals a little more slowly..... it just doesn't get hammered into shape as sharply or quickly..... and the pressure jet escaping past the bullet is venturi'd down such that it actually increases in temperature as it's flowing down the grooves at the corners of the lands. Pop the pressure-sensing hose off on your air compressor to see this effect in action. The little copper fitting will get hot enough to melt and start your shop on fire in real short order!


Incidentally, many believe that copper fouling is primarily caused by just this escaping or leaking gas jet which melts the copper and re-deposits it on the bore surface near the throat and again at the muzzle. The center portion of the bore remains nearly copper free.

I personally believe that rifling profile, contour and finish as well as the angle of the leade and the method used to profile the lands/grooves (cut VS button-swaged or broached) all work to produce different results with different barrels/bullets/calibers. Some makers' barrels just "don't copper as much" as others..... but quantifying the perfect blend through the different calibers, rifling styles and cartridges is problematic. For myself I've had better luck with certain combinations. I've had different reamers (same basic cartridge) which seem to work better with certain brands/styles of bullets/barrels. All anecdotal info not worthy of sharing.

I will say this though, IMO if a winning builder recommends a certain brand or style of barrel and bullet for use with his reamer it's worth listening to him.

An awful lot of this stuff is just "results based on trial and error" and will remain so unless someone devotes tremendous resource to studying and attempting to quantify things not really worth the time and expenditure.

BTW Harold Vaughn's book "Rifle Accuracy Facts" is a great place to start learning about the parameters involved.

hth

al
 
The only scientific studies on predicting erosion that I've found online have been directed at military cannon rather than rifle calibers but the principles remain basically the same.

Gas Erosion is a two part situation, Thermal Gas erosion and Gas/mechanical due not to powder grains but rather the molecular weight of the gases of combustion of various propellants.
The heavier the molecule the more efficiently it transmits heat/energy to the surface it strikes. Damage to bore steels is deeper.

Double base powders containing temperature moderators that reduce flame temperature to aproximately the same levels as single base powders will continue to erode a bore slightly more quickly due to the heavier molecular weight of the gases produced by Nitroglycerin. Carbon Dioxide VS Carbon Monoxide for example and a string of heavier nitrogen oxides and peroxides.
Velocity of the gases independent of barrel time due to ocilation also contributes to not only increased erosion but also to gas erosion appearing in portions of the bore further forward than erosion from single base powders, the latter more often confined to the first few inches of travel.

The superheated particulates, especially the non combustables and those combustables that can't oxidize till they reach open air as muzzle flash, also transmit heat to the bore surface. The more non energetic additives a powder contains the greater the effect.
 
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The first word of the initial posting sort of threw me -- "obviously." I am not sure that any of this is all that obvious.

Some issues to look at -- the idea that shooting out of a hot barrel will degrade the barrel. How hot is hot? Based on touching the barrel with your hand, it may feel quite hot to you -- but temperatures that soften steel are probably beyond what we range shooters are ever going to encounter.

Steels -- chromium steel, moly steel, stainless steel, high-carbon steel. They would seem to differ in their ablation rate-- or whatever we call barrel degradation. Is it valid to make broad generalizations? Even button vs. cut rifling-- the friction of bullets traveling down the bore may differ.

Shoulder angle of casing -- for years I have read that the 22-250 erodes throats less rapidly than the 220 Swift because it has the rather steep 28 degree shoulder. Now I guess that's out the window.

This whole discussion reminds me of the health warnings of the 1960's. Coffee is good for you -- no, bad for you. Eggs are good for you . . . no, bad for you. Splenda is good for you . . . no, bad for you.

For a simple man like me, just shoot them until they wear out. Now that isn't too hard to understand, even for me.
 
Mirage416 said:
If steel is eroded by the gas which is forced to converge upon the throat, how much is the softer brass in the cases eroded in the neck area upon each firing? Has anyone found brass to erode or thin more than they desire in the amount of firings the brass is typically good for? Or maybe this itself is what actually constitutes the life of the brass. :confused.
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Brass may be eroded as much as barrel steel, but few people try to use a single piece of brass as many times as they use a barrel. If you reload 50 pieces of brass for one rifle that's 1/50 the exposure to erosion the barrel gets. Brass has higher thermal condutivity but a lower melting point than barrel steel. I don't know which is more important to the erosion of brass.

My experience is that brass work hardens and cracks before it shows significant erosion. I don't consider throat erosion a problem. Replacing barrels is the price of shallow trajectories and low wind deflection. The alternative is to move closer to the target.
 
Actually it's entirely possible to use brass until it shows neck wear, thinning. I reload cases 50-100times without annealing (No, the necks do not crack) and old brass does get thinner.

Whether this is from abrasion of the reloading die or ablation I don't know.

But Louis' right.... the cases will never get the wear the barrel gets.

al
 
Montana Pete said:
The first word of the initial posting sort of threw me -- "obviously." I am not sure that any of this is all that obvious.
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I can see that there are many more underlying factors than I had orginally imagined. :eek:

About the brass wear question. I realize each case is used a smaller amount of times than the throat. But the case is the "nozzle", and where the gas is initially forced to converge into the neck. So I wondered if that made erosion apparent over the short number of firings in its lifespan.
 
A few tidbits of information you might find useful.

Blowby accelerates gas erosion, usually the lands are less affected than the grooves.
A boatail bullet tends to allow more blowby than a flat base bullet. The flat base, especially exposed lead base military type FMJ tends to fill out the grooves better more so once the bottom of the grooves have begun to wash out.

The British noted this when Mk8Z ammunition with boat tail bullets propelled by NC powder was fired in a Rifle barrel that had noticable gas erosion from MkVII ammunition with hotter burning Cordite.
The throat of the rifle would have significant erosion after around two hundred rounds of cordite loaded ammo.
The flat base MkVII bullet slugged out enough to form a tight seal but the Mk8 bullet did not.
Once the erosion from cordite had reached a certain level gas blowby around the Mk8 bullet was far more than it would be in a bore not eroded by cordite, further eroding the throat of the rifle.

The Mk8 bullet designed for extra long range indirect fire from heavy machineguns has a long tapered boat tail , far more radical than that of most such designs.

Similar instances of increased gas blowby have been studied with bullets of less radical shape.
When the grooves are washed out or the bullet too hard to slug into the grooves blowby can heat the projectile (in cannon its said the projectile can become heated to "incandesence") damaging the bullet jacket and leading to jacket failures.

Another tidbit
Some old time long range shooters prefered to set back and rechamber a barrel with eroded throat rather than rebarrel.
Mechanical erosion from bullet travel apart from gas erosion, tended to leave the lands worn fairly tapered, the dia across the lands a hair larger at the leade than nearer the muzzle, the grooves generally deepened spmewhat by gas erosion. The result being a tapered bore. As the bullet traveled down the bore it made a tighter fit as it went, reducing blowby and giving the lands an increasingly better grip on the bullet.
Barrels worn smoothly enough, and undamaged otherwise, were often more accurate after being set back and rechambered than they'd been when new.
 
How can throat wear be predicted?


If you shoot it, it will come!:)
 
big case small bullet and vel., over 3000fps spells rapid throat erosion. Just about the time you break-in the barrel and get'er tuned with your favorite load or two it all ready begun. ! :(
 
speedpro said:
big case small bullet and vel., over 3000fps spells rapid throat erosion. Just about the time you break-in the barrel and get'er tuned with your favorite load or two it all ready begun. ! :(
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Sounds like what I posted earlier would apply in that situation.

Another tidbit
Some old time long range shooters prefered to set back and rechamber a barrel with eroded throat rather than rebarrel.
Mechanical erosion from bullet travel apart from gas erosion, tended to leave the lands worn fairly tapered, the dia across the lands a hair larger at the leade than nearer the muzzle, the grooves generally deepened spmewhat by gas erosion. The result being a tapered bore. As the bullet traveled down the bore it made a tighter fit as it went, reducing blowby and giving the lands an increasingly better grip on the bullet.
Barrels worn smoothly enough, and undamaged otherwise, were often more accurate after being set back and rechambered than they'd been when new.
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Sounds like the same procedure might benefit BR shooters.
A bull barrel would be better suited to setting back, a tapered reinforce of a sporter or military profile barrel could only be ste back a very little but a bull barrel could be set back as much as needed, till it got illegally short at least.

PS
Some time back a military shooter recounted in a post on another board how a lot of Sniper Rifle barrels had been ruined by an armorer who tried to speed up the breakin period.
He'd been told that the rifles required 60 shots to breakin to achieve optimum accuracy. These were chambered for one of the long range magnum sniper rounds.
He fired 60 rounds from each rifle as fast as he could reload and they then found that all the rifles had been badly eroded and could no longer group within specs.
All these rifles had to be rebarreled.
 
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