barrel erosion and powder types

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Quiet

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I have just been reading an interesting, but rather old (1930's) book on ammunition and ballistics.

In comparing powder types, this book says that double base powders have some advantages over single base powders in that they can produce more velocity by weight and they are less hygroscopic (less likely to absorb moisture). The source also states that the nitroglycerine content of double-base powders means that they burn at a hotter temperature than single-base powders and therefore contribute to greater bore erosion, reducing the service life of weapons and for this reason double base powders are not used by the US military. The same source also says that the high temperature of double base powders caused problems with excessive flash because it is not practical to add the amount of inert additives necessary to cool the gasses enough to eliminate the flash.

Now, this is an old book, and I know that ball powders have been in use by the military since the 50's (although that is about when chrome-lined bores came into use), and presumabably they have also found additives to control flash, in spite of the heat. Has the barrel erosion problem been solved as well?

I usually shoot single base stick powders like 4895 and Varget, and have never shot out a barrel but as a high-volume reloader, the advantage of easy metering ball powders and slightly lower cost makes them seem very attractive.

Have any of the target shooters on this board who track the number of rounds shot and shoot barrels enough to wear them out notice a difference in barrel life between double-base powders and single-base powders?
 
Ball powders are easier and cheaper to manufacture. That's their only advantage. They meter more accurately, too, but measuring is still more fail safe. Extruded powders are more physically consistent, tend to be cleaner and are usually more temperature stable. IMHO, they're the only viable choice for rifle ammo.
 
Powder burning and heat are not the primary factor of barrel erosion. Type of powder has no effect on erosion either.

Read Chapter 8 of this.
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I read all of chapter 8, and it does elude to some minor contributing factors such as the cyclic stress (fatiuge) ultimately producing micro cracking and attendant hydogen and nitrogen embrittlement resulting in some exfoliation. The major cause of throat erosion is what he incorrectly refers to as "ionization". For metals, ionization is the loss of electron(s) producing a positively charged species. The correct term for the process he is describing is sublimation. Sublimation is the transformation of solid phase material to the vapor phase. With the flame temperature of double base propellants being greater than 2500 C (melting point of steel approx. 1500 C), the outer most layers of atoms in the throat area receive enough thermal energy to be vaporized without melting of the underlying steel. It is not pressure which is responsible for this sublimation process (greater pressures favour the solid phase, not the vapor phase), it is temperature (thermal energy) which drives sublimation. The large capacity cases in conjunction with small bore diameters certainly exacerbate the erosion process, as more thermal energy (larger powder charge) is forced through a smaller aperature (bore) resulting in the outer atomic layers of the steel in the throat area to attain more than sufficient energy to leave the solid phase. The throat area is subject to the greatest erosion as it experiences the highest temperaures for the greatest duration in comparison to the rest of the bore.
 
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I was always under the impression that ball powders were double base powders, but doing some more reading it seems this is not the case.

It appears that the double base powder this reference was talking about was cordite.
I can't find any powder listings that describe a modern rifle powder as double base, so I guess this is a non-issue.

It still leaves me wondering what powder I should use to get maximum service life for the $$$$ I sunk into a 6.5-284 barrel.
 
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There is also the theory that slower burning powders sandblast the throat with particualte.
Myself, I think the average shooter isn't going to see much in his lifetime, at least on a modern firearm.
Certainly some of the gunnutz here never will see it, because they never own a firearm longer than a few weeks. :D
 
Quiet said:
I can't find any powder listings that describe a modern rifle powder as double base ...
Click to expand...

Quite a few are. The Lyman reoading manual has a powder section that describes the shape (extruded, flake, spherical) and type (double- or single-base).

If you're varminting with an 'overbore' cartridge like 22-250 (I think that's the term), then a less-hot powder might help out. I've never seen a side-to-side study - no surprise there. Imagine the cost and time required.
 
Today there are many double-base powders in regular use out there. Alliant has been mentioned already, Norma is another. The Vihtavuori N500 series powders are all double base, as are many ball powders. These are nowhere near as hard on barrel steel as was Cordite, which often contained up to 25% Nitroglycerine. Coatings have also improved significantly. I doubt that you could tell the difference in barrel life using single base or double base modern powders. Use what works for you and ignore the noise made by the speculators and doomsayers. Regards, Eagleye
 
downwindtracker2 said:
I as I understand it,as the pressure goes up so directly does the temperature.
Click to expand...

I'm not shure that the burning temperature of the powder has anything to do with the pressure of the load, then again I might be wrong :confused:
bigbull
 
bigbull said:
I'm not shure that the burning temperature of the powder has anything to do with the pressure of the load, then again I might be wrong :confused:
bigbull
Click to expand...
There may be a correlation for the specific powder being used, it would be hard to find something like that across the broad spectrum of powders in use today.

P.O. Ackley was a believer in the sandblasting theory. The cause is really of little consequence to us, just something to watch for on a used gun purchase.
 
Quote:
Originally Posted by downwindtracker2
I as I understand it,as the pressure goes up so directly does the temperature.


I'm not shure that the burning temperature of the powder has anything to do with the pressure of the load, then again I might be wrong
bigbull
Click to expand...

A simplified approximation of the relationship between temperature and pressure can be described by the ideal gas equation:

PV=nRT rearrange for pressure as the dependent variable, gives:

P=(nRT)/V

where: P= Pressure, n= moles of gas, R= universal gas constant,
T= Temperature (absolute scale), V= Volume.

Looks like a simple equation, however with the dynamics of firing, the only two constants are the universal gas constant, and the number of moles of gas generated by the fixed mass of the powder charge. The pressure, temperature, and volume behind the bullet are continuously changing as the bullet is accelerated down the bore. Solutions involve more calculus and thermodynamics than any of us care to entertain.:evil:
 
Simplifying the nerd-speak - if you take a volume of gas - say one litre - and compress it suddendly and violently, it'll get HOT. The reverse is also true - if you take a volume of compressed gas and decompress it suddenly, it'll get cold. This is how refridgerators work - compressed hot gasses on the outside and decompressed cold gasses on the inside.

This is why those propane cylinders get all frosty when you shoot them and they decompress rapidly :D

So inside the gun, you have the temperature of the powder combusting, and the heat from the gas compressing rapidly (as the combustion proceeds it produces moregas in a confined area, thus compression). This heat should be balanced by the decompression when the bullet leaves the muzzle, so the NET heat added to the barrel is solely that from combustion and maybe friction. But there is a brief period when there is significantly MORE heat in the bore than that of combustion alone
 
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