Bore erosion , shed bullet jacket, online data?

[Old Gunner]

I'd tried to explain to some folks on another forum how gas blowby damages bullet jackets and how unequal areas of gas erosion can lead to the jacket of even FMJ being torn away from the core and lodged in the bore.

This was something I'd first learned of many years ago in reading the expert testimony in a case of criminal negligence where a gunshop owner had examined a rifle barrel and told the owner it was safe to shoot.

This level of gas erosion is seldom encountered these days, the rifle involved in the court case was an antique that that been used with high temperature propellants , mostly cordite.
Problem is that due to estate sales and such a suprisingly large number of antique rifles are now being sold through online auction sites with the buyer having no real knowledge of the types of ammunition used by those rifles when they received the vast majority of their use.

Heres what I remember of the testimony in the case I mentioned.
The Rifle had been fired with Cordite, to the unaided eye the rifling looked good and the bore not excessively rough or pitted.
When the owner testfired it the jacket and probably part of the core remained in the bore. The core or most of it must have exited the muzzle so the shooter thought that the bullet had gone through, he tried another shot.
The second bullet telescoped into the remains of the first, excessive pressure blew out the bolthead and a part of it including the claw extractor entered the shooter's body severing a large vein.

The folks on this other forum have convinced themselves that gas erosion can not produce a dangerous situation in a rifle bore. They believe that all propellants erode bores in the same pattern, gradually from breech to muzzle.
They don't recognize that double base propellants can erode bores at a much greater rate than single base propellants when no moderating additives are present, and due to molecular weight of gases of combustion even heavily moderated modern double base powders will erode slightly more quickly than single base powders at the same flame temperatures.

I've linked to PDF files from the Australian Ministry of Defence and other scientific studies on gas erosion in cannon tubes but they ignore these claiming that these aren't relevant to small arms.

What I really need is a source of up to date information on the erosion of rifle bores and how bore condition can cause increased pressures.

What makes judging gas erosion more difficult when higher temperature propellants such as Cordite were used is that this sort of erosion washes out the grooves at a faster rate than the lands, so a bore may look nearly normal to the unaided eye yet be several thousandths over size in the major diameters.
Another problem is the wide manufacturing tolerance of groove depth a century ago. A tight bore can fire several times the number of rounds before it wears enough to reach the tipping point, than a bore that already had overly deep grooves when it left the factory.

I strongly suspect that gas erosion was the root cause of the disasterous breech failure that killed Glenn deRuiter.
His Lee Navy rifle would have used a very high temperature Double Base propellant when in service.
When a load known to be safe shows excessive pressures the first likely suspect is bore condition.
Ejected cases of rounds fired before the fatal shot showed flattened primers according to a witness.
No way of knowing at this time, but gas erosion was common with the Lee Navy.

It seems counterintuitive that a loose bore could result in higher pressures, I understand the mechanism, but can't explain it properly without a reputable modern source that deals directly with the problem in rifle bores.

 

[alinwa]

I think you're all wet on this one. The other guys are right.

I've worn out some barrels. Lots of guys on this board wear out many barrels per year.

IMO gas erosion CANNOT cause erosion such that a bullet jacket would be left in the bore because of it, and there is NO evidence that "a rough and eroded bore increases pressure" as is commonly claimed.

"Gas blowing by the bullet" is not what erodes bores in the first place. Microcracking caused by differential temperature extremes is the root cause of most erosion. This microcracking produces thin edges which are then more easily ablated (gasified) by the presence of high temperature gas present during the combustion cycle. This gasified metal is then carried out the bore as ejecta.

Nor does gas blowing by the bullet scarify, erode nor degrade the integrity of the bullet itself. It doesn't have time to. Go out this winter and fire some bullets into a snowbank, recover them. Take pictures, show the erosion. I've got pictures, NO erosion.

a rough bore may well ABRADE the bullet jacket, ripping it such that it blows apart from rotational or "hoop" stress..... after it leaves the muzzle.

I find the conjecture that a bore can erode so far that the bullet "reaches a tipping point" to be just silly, certainly unsupportable.

If you understood the mechanism you could explain it.

al

 

[Old Gunner]

I think you're all wet on this one. The other guys are right.

I've worn out some barrels. Lots of guys on this board wear out many barrels per year.

Not likely that you'd wear out a bore to the level of erosion seen in many antique rifles, or that you use propellants with flame temperatures approaching 5,000 degrees.

IMO gas erosion CANNOT cause erosion such that a bullet jacket would be left in the bore because of it, and there is NO evidence that "a rough and eroded bore increases pressure" as is commonly claimed.

Well if you want to go on record with that statement far be it from me to correct you.

"Gas blowing by the bullet" is not what erodes bores in the first place. Microcracking caused by differential temperature extremes is the root cause of most erosion. This microcracking produces thin edges which are then more easily ablated (gasified) by the presence of high temperature gas present during the combustion cycle. This gasified metal is then carried out the bore as ejecta.

Thats part of the process.

Nor does gas blowing by the bullet scarify, erode nor degrade the integrity of the bullet itself. It doesn't have time to. Go out this winter and fire some bullets into a snowbank, recover them. Take pictures, show the erosion. I've got pictures, NO erosion.

I'm sure all those stuck bullet jackets that resulted in specialized tools to remove them were the result of pixie dust or alien mind control then.

a rough bore may well ABRADE the bullet jacket, ripping it such that it blows apart from rotational or "hoop" stress..... after it leaves the muzzle.

I find the conjecture that a bore can erode so far that the bullet "reaches a tipping point" to be just silly, certainly unsupportable.

If you understood the mechanism you could explain it.

al

The erosion reaches the tipping point, as in suddenly begins to procede at a greatly accelerated rate, not "the bullet"reaches a tipping point".
my words were
"What makes judging gas erosion more difficult when higher temperature propellants such as Cordite were used is that this sort of erosion washes out the grooves at a faster rate than the lands, so a bore may look nearly normal to the unaided eye yet be several thousandths over size in the major diameters.
Another problem is the wide manufacturing tolerance of groove depth a century ago. A tight bore can fire several times the number of rounds before it wears enough to reach the tipping point, than a bore that already had overly deep grooves when it left the factory."

An eroded throat can also result in the bullet being slightly tipped in relation to the origin of rifling, but throat erosion itself is not the main element I'm interested in.

You have provided a classic example of "if I ain't seen it it can not exist".
The nay sayers are also hard to convince because their experiance has been limited to best case scenarios.

Gas blowby damaging bullet jackets is a known phenomena, not a theory. If you've never owned a rifle that had this level of erosion then you can't really say what can happen in a bore that has it.
Severly undersized bullets that are too hard to upset enough to fill out the grooves can also be overheated even when erosion isn't present.

The mechanism is easy enough to understand and there are authoritative studies , but those easily available on the net deal mostly with larger caliber weapons 20mm and up. I'm looking for one that deals with more common rifle calibers.

If you wade through dozens of works of the ealy 20th century the information is there, but not in easily digestable form.

A bore worn beyond the accuracy level one user requires may still exhibit usable accuracy for another user with different requirements.
A target shooter, or a hunter who's standards of accuracy at long range are high will discard a worn barrel long before it reaches the level of erosion too often found in older rifles.
Age of a rifle doesn't automatically translate into the level of erosion you can expect to find. Number of rounds fired, rapidity of fire, and propellant type are the relevant factors, with resistence to erosion of the alloy used in manufacturing the barrel an important consideration as well.

Early double base propellants used far higher Nitroglycerin content (up to 58%) than todays Double Base propellants (less than 10%).
Some milsurp ammo with Nitro content of 30% or greater is still to be found in quantity.
Moder DB propellants NC/NG contain temperature reducers that bring down the flame temperature to much the same level as Single base NC propellants, but the greater molecular weight of the gases of combustion and their velocity, especially when those gases oscilate towards the end of their burning time, and the superheated particulates of non combustable additives, results in a higher erosion rate than NC.


I'll add some quotes from authorities on the erosion characteristics of propellants when time permits.

Understanding how something works doesn't always mean it can be easily explained.
The research was done many years ago, finding an easily understood authoritative source is what I'm after. Preferably one with cutaways of eroded bores.

Heres a more modern source
http://www.riflebarrels.com/articles/barrel_life3.pdf

The problems of erosion and blowby in rifle caliber barrels was worked out so long ago that there don't seem to be any recent studies other than those dealing with heavier caliber guns.
Most modern rifle barrels and rifle propellants have been updated in metalurgy and chemistry to the point that mechanical erosion is likely to end the life of the bore long before erosion reaches the levels found in many antique rifles.

The high Nitroglycerin content propellants are seldom encountered except in ammunition from the 1960's or earlier. Some of this ammunition is still on the market and usable so long as the primers are still active.

 

[Donald]

OG,
I suppose in some rare cases one might find a bore so rusted, worn, neglected, corroded, etc that what you say might happen. Great big "might" there. I think the chance of that happening might be on the order of one tenth of one percent. No, less than that even. To strip the jacket off and leave the lead core is unbelievable. Along with that goes....who on earth would try to shoot a gun with a barrel in that condition. Two things could contribute to it. Ignorance and stupidity. Ignorance is the lack of knowledge and can be corrected. Stupidity is to the bone and is not correctable. And I bet we all know at least one stupid person......Donald

 

[Old Gunner]

OG,
I suppose in some rare cases one might find a bore so rusted, worn, neglected, corroded, etc that what you say might happen. Great big "might" there. I think the chance of that happening might be on the order of one tenth of one percent. No, less than that even. To strip the jacket off and leave the lead core is unbelievable. Along with that goes....who on earth would try to shoot a gun with a barrel in that condition. Two things could contribute to it. Ignorance and stupidity. Ignorance is the lack of knowledge and can be corrected. Stupidity is to the bone and is not correctable. And I bet we all know at least one stupid person......Donald

Pretty much my point. I've tried to impress on these guys that bore condition is very important to safety, they even argued against test firing antique rifles remotely before firing them from the shoulder with full power loads.
I examined a bore once that had a large chunk of one land broken away, and it took some doing to convince the owner that this rifle should be relegated to wallhanger status. The bore was so eroded that diameter in some sections was greatly over sized. Rifling at the muzzle still looked okay, gas erosion seldom is visible at the muzzle, the damage being done long before that.

Another fellow insisted that the bore of his Japanese rifle was perfect, It turned out to be a blank firing drill rifle with heavily pitted smoothbore barrel.
It took some convincing to prevent him from trying to fire live rounds in it.

Anyway heres some information on damage to bullet jackets due to blowby in heavily eroded bores.
From the West point academy "Cadet Gunnery and Ordnance Manual 1914"
When high temperature DB propellants were still being used.
The worst offender was the early HiVel with .30% Nitro, and later HiVel2 with around 20% Nitro.
http://books.google.com/books?id=QT...kqGyCQ&sa=X&oi=book_result&ct=result&resnum=1

The accuracy of the rifle is principally affected through the erosion of the barrel by the powder gases. The gases, highly heated and moving with high velocity under great pressure, attack the Avails of the bore, which are probably softened by the great heat, and cut irregular channels in the metal, destroying the surface of the bore and the rifling. The erosion is greatest at the seat of the bullet immediately in front of the cartridge case, and extends forward into the barrel for several inches. Beyond this the walls of the bore are practically unaffected.

The effect of erosion is well shown in the enlarged photographs, Fig. 266, of rifle barrels from which 3500, 4000, and 5000 rounds have been fired.

When the erosion has become marked, the bullet is forced against an irregular surface and the metal of the bullet jacket, probably also softened by the heat, is unequally stretched on different sides, producing a decided eccentricity of the point of the bullet and great irregularity of the base. The sides of the bullet are deeply scored by the powder gases escaping past the bullet and by the irregularities of the bore.

In Fig. 267 are shown enlarged photographs of a service 220- gr bullet, model 1903, recovered after being fired into sawdust from a new rifle barrel, and of bullets fired from barrels that had been previously fired 3500 and 4500 times.

The deformation of the bullet is the chief cause of its inaccuracy. At the same time its muzzle velocity is reduced by the escape of the gases past the bullet in the bore.

Velocity And Pressure.—The erosive effect of the gases appears to depend more on their velocity than on the maximum pressure. Thus in tests that were made with the sen-ice rifle with 220-grain bullets fired with muzzle velocities of 2300 and 2200 feet, the maximum pressures in the two cases not being very different, the first appreciable falling off in accuracy occurred after 2000 rounds with the 2300-foot velocity and after 4000 rounds with the velocity of 2200 feet; and the accuracy after 7000 rounds with the lower velocity was better than after 4000 rounds with the higher.

Ammunition loaded to produce a muzzle velocity of 2300 feet was originally used in the service rifle, but after the above mentioned tests the muzzle velocity was reduced to 2200 feet and the accuracy life of the rifle increased from 2000 to 4000 rounds.

The set of photos shows increasing damage to bullet jackets due to Blowby as the test barrels were subjected to thousands of rounds of the high temperature powders.

At that time a special tool for extracting shed jackets was issued. It was a plug shaped like the base of a fired bullet, with grooves and lands cut to fit the grooves of the bore, so equal pressure could be applied all around.

Appendages.—Among the appendages provided for the care of the piece is a bullet jacket extractor, Fig.270, a cylin- drical steel plug rifled on the ex- Fra. 270.

terior to fit the bore. This is pushed down the bore from the muzzle until it rests on the bullet jacket, which may then be forced out of the barrel.

When IMR single base powders were adopted the issue of the jacket removal tool was greatly reduced in numbers and restricted to certain personel best qualified in its use.

The .303 British military ammo used a card wad between bullet and high temp Cordite charge. This helped seal the bore from Origin of rifling to the point where the edge of the card was worn down and lost its ability to seal. With that sort of ammo there can be secondary areas of erosion mid way of the bore, often worse than the erosion at the throat. This can produce a very dangerous condition.
Since grooves are washed out at a rate near to or even higher than the lands a plug gauge can only measure wear to the lands and the effective bore diameter can be far oversized while the bore still passed casual inspection.
Mechanical erosion wears away the lands faster than the grooves.

PS
From the Report of the Chief of Ordnance
War Department Annual Reports 1905

SMALL-ARMS AMMUNITION.

Ball cartridges^ models of 1898 and 1903. — The thickness of the
metal of which the bullet jacket is made was increased to prevent,
as much as possible, the stripping of the jacket from the lead core in
eroded barrels. A machine is being designed to form the bullet cores
from wire-drawn lead rods, as it is believed this method will result
in greater uniformity in the weight of the bullets.

http://books.google.com/books/downl...DMSA-lR5vjLy_bxFgA&source=gbs_summary_r&cad=0

So this statement by Alinwa

IMO gas erosion CANNOT cause erosion such that a bullet jacket would be left in the bore because of it,

Is proven wrong.

Also further along in the report they detail an experiment in determining the amount of force necessary to force a bullet down the bore of barrels at different stages of erosion. The force needed increased greatly as the level of erosion increased.
This was done using mechanical means but it suggests that chamber pressures could be greatly increased in eroded bores.
The amount of force varied greatly at advanced stageas of erosion. One bullet might require little extra force while the next might require extreme force to move it along.