Home Forums Hunting & Shooting Ask The Gunwriters What creates barrel heat: velocity or pressure?

Dr Howell
I have No. 599 of your book re Custom Cartridges. Do you have another available? When will we see the second volume? Thanks

No second volume. The ol' fa� � uh � fossil just can't do it.

Expect to hear soon that the first volume will again be available (but not from me). I gave the films to a friend, and he's reprinting it. I don't (and won't) have the capital.

I'll let everybody know, as soon as I hear.

I've designed a passel of cartridges since I did that book. May consider offering a collection of those drawings. Will also continue to design a few custom wildcats now 'n' then, if anybody wants me to.

"ad nauseam: Definition from Answers.com
ad nauseam adv. To a disgusting or ridiculous degree; to the point of nausea. [Latin ad , to + nauseam , accusative of nausea ,"

Ken, thank you!

SU35, you've asked two separate questions, and Ken has tried to explain that no single answer can cover both.



Yes, I think the lower pressure cartridge will give longer barrel life due to the lower gas temperatures. There may be more total gas at that lower temperature; however:



This one's not so easy. Ten gn more powder producing that much less pressure is likely not burning fully. Those bits of unburned powder represent heat not liberated flying out the barrel with little effect on heating the barrel beyond rubbing. All those lumps of unburned powder banging on the heated barrel throat could be erosive; I can't say for certain.

If all the powder yet manages to burn, there will be more gas but at a lower peak pressure and temperature. The total heat dump to the barrel is a combination of the total flow and the flow's temperature and pressure. It's not obvious to me how the combined effect will vary.

Also, a long burn will be releasing some heat down the barrel instead of all inside the chamber. This will spare the vulnerable throat some heating. The throat is the spot usually damaged. It sees the highest temperatures for the longest time.

Air guns-- significant amt. of heat is generated by an air gun. One can use the ideal gas law eq. (as stated earlier) pv= muRT to or pv/t=p'v'/t' show the algebraic relationship between the variables, or simply leave some oil in the barrel, shoot it and as it diesels conclude this as the gun falls apart.
The amt. of temperature differential between say two of the same wt. charges but different burning indexes is a funtion of mu in the above equation, the molar mass of the nitrates used in the propellant. As faster powders usually contain a higher ratio of nitro, the temperature will be higher. but for the grass roots guys use the same charge of 296 vs.231 in your 44 mag. and conclude this as the gun falls apart. (this can also be seen as a broader curve in the peak pressure curve if you have something like quikload, or Matlab and pressure equip).
The particles of heated material from the powder, (or corn cob if you forgot to shake the case out) don't simply rush out the end of the barrel (at least while the bullet is still in the barrel) the kinetic energy they transfer to a specific location on the barrel metal in available while the bullet is still in the barrel. Think plasma torch.
The amt. of energy from friction is insignificant compared to the other factors. Remember how it easy it was to get the bullet out with the cleaning rod when you forgot the powder and the primer pushed it half way down the barrel.
Barrel life, or more accurately lead life, or more accurately accuracy is caused by the cooling off of the barrel, and the cracking that occurs than the heat of the barrel, per se. Granted the barrel needs to be heated in order to cool off, but its good to map causality.

BTU and temperature are very different units. BTU is a measure of kinetic energy. One BTU is the amount of energy required to raise the temperature of one pound of water by one degree fahrenheit. Temperature is a measure of a physical property of an object. It does not indicate the amount of kinetic energy or work that can be accomplished because temperature includes no measure of capacity.

A couple years ago I experimented with loads in my 45-70s using a few different powders. Medium velocity loads with Reloder7 powder didn't seem to heat-up the barrel much at all (and were extremely accurate, by the way). However, lower velocity loads with much lighter charges of XMP5744 powder seemed to heat-up the barrel much more quickly (higher temperature with fewer rounds fired). This seems to fly in the face of the idea that larger charges of powder, or hhigher chamber pressures, create higher barrel temperatures as a rule. A bit of checking revealed that 5744 contains a relatively high percentage of nitroglycerin as compared to Re7. I always wondered if that is the reason for the higher temperatures with 5744. Any insights?

The amt. of temperature differential between say two of the same wt. charges but different burning indexes is a funtion of mu in the above equation, the molar mass of the nitrates used in the propellant. As faster powders usually contain a higher ratio of nitro, the temperature will be higher. but for the grass roots guys use the same charge of 296 vs.231 in your 44 mag. and conclude this as the gun falls apart. (this can also be seen as a broader curve in the peak pressure curve if you have something like quikload, or Matlab and pressure equip).

A related issue regarding barrel heating due to friction came up with the adoption of the M249 LMG. The guns would develop unexpectedly wide dispersion as they heated up during testig at maximum sustained rates of fire. The problem was determined to be related to the "buttering" of the then lead core bullets. The bullet would be heated to the point where the core would get soft when the gun was fired at near maximum sustained rates of fire for extended periods of time.
I seem to remember a discussion of the measured internal barrel temps showing highest temps at the throat which generally decreased as you went towards the muzzle as you would expect in terms of time exposed to the high pressure gas. (The barrel steel near the muzzle is exposed to the hot gas for an extremely short period of time.)However, the temps began to climb again fairly dramatically towards the muzzle again due to the heat from the friction of the high velocity 5.56 bullets at high sustained rates of fire.
The cure for the buterig problem was adoption of the green tip, non-lead round for the 5.56.

For the record, DO NOT USE THE SAME CHARGE OF 231 POWDER VS. 296 POWDER IN YOUR 44 MAG! This will surely cause dangerous pressure, damage to the gun, and possible injury to the shooter, up to and including death. I know that Etoh didn't really mean to suggest that, but it seemed to me prudent to reiterate the point. There is an example of just such a mistake hanging on the wall at Jay's Sporting Goods. It used to be a Ruger Super Blackhawk. Now it is just twisted metal with major pieces missing.

Etoh; Did you allow for the energy to make the gun fall apart?

not sure what the question is redhead, my example was simply a statement on differences in molar content of nitro between two powders when compared on a wt. basis. The potential energy stored up as chemical energy in the form of the powder must be above the "burst" limits of the device regardless of the pressure. On my NFA weapons, prolonged periods of full auto and barrel muzzle temp. increase especially when using a suppressor are quite noticeable. However if you wish to include these in the discussion , we should bring in artillery also as the friction plays an even profounder role. Any "model" verbal or mathematical must be explained in the basic gas laws to be relevant or it becomes a series of testimonials.

How warm a barrel feels to the touch is not (necessarily) what causes barrel erosion. I could pass 250 F steam all day through a stainless barrel, and it wouldn't erode, but you certainly couldn't hold it in your hands.

To erode, the surface in the bore has to be brought to a high enough temperature and bombarded with gases and propellant particles. I simply don't know what combination of temperature, pressure, velocity, and duration are required. While the combination of temperature and pressure is a major component of the heat transfer to the throat, the total volume of gas passing by is also significant. In the example cited by SU35, the temperature will be much lower, but the volume is (perhaps) higher.

As you noted, it's not just grains of powder that count; the energy per grain is important. Also important is how the burn progresses. A slow, low pressure burn leaves more energy in the gases. The slower burn will also leave the barrel corked longer by the bullet, so while the peak gas temperature might be lower, the total heat transfer to the barrel could be more. Again, I don't have enough numbers to help here. A full 3D simulation of the problem is probably the best way to learn, but only the Army is running those.

All by-products of internal combustion Heat, pressure, friction but no need to make it any more complicated than it is

one left out force here is torque. It's more informative to do this problem in units of work, How much work does it take to heat a barrel to, so, and so, how much work is created by the torque. you could do a finite element analysis on the problem easily in matlab, but as stated -- if its that important someone has to pay.
each time the barrel cools down from the heat the cracking differences in cooling increases the surface area open to erosion by the next shot.

Original question wasn't a request for anyone to do an energy balance...

Most of this has been touched on already, but...here are the ways heat energy enters barrel steel during a shot:

1. Bullet passage down bore: heat energy into barrel equals part of the friction force times distance travelled (bullet gets some too, but I don't know if it's exactly half/half).

2. Not-quite-elastic springback of barrel steel after barrel dimensions change ever-so-slightly in response to containing tens of thousands of PSI. The stress and resulting strain developed in the barrel steel can be calculated, and I presume there's some factor out there to account for damping/energy loss in springback. Can't remember offhand.

3. Convection between hot combustion products and barrel insides.

4. This one is out there a bit - if you're flowing fluid through a conduit, there is friction between the flowing media and the conduit. That means that pressure energy in the flow is dissipating as heat either in the flowing media or the conduit. I'd bet both.

the op question was answered with the poster that gave the pv/t=p'v'/t' equation. the relationship is linear, after that it was thrown out to open discussion

the amt. of friction is really small compared to the other forces, if a primer can push the bullet half way into a barrel,no where close to half and half. the heat erosion at the base of the bullet will cause "buttering" if the base is soft enough.(cast bullets at higher velocities).
#4 isn't that far out. Rheology physics usually assigns a viscosity constant to the fluid however, in this case it would be difficult.

It's a good question, because you have two effects operating at the same time, one pulling one way and the other pulling the other. It's probably difficult to determine without experimentation, and it's a lengthy experiment. I doubt anyone is going to give a numerically correct answer from first principles.

PV=NRT only works for an ideal gas. The gas in the barrel of a gun is far from ideal. Even pure Nitrogen is non-ideal, because it comes in two-atom molecules. However, the pressure/volume/temperature ratios still work.

The passage of the bullet down the barrel is not quite an adiabatic process. A cold barrel will "rob" more energy out of the propulsion gas than a hot one does, because heat transfer is proportional to temperature difference, and there is less temperature difference between a hot barrel and the hot gas. The dominant reason guns shoot faster in hot weather is that the barrel is hotter and steals less propulsion energy. Temperature of the ammunition is a secondary effect. It has practically nothing to do with the initial temperature of the powder. It's mainly steel, brass, and lead absorbing heat from the gas.

and an oxy acetylene flame is around 6000 degrees F.

apply the flame of a torch to a barrel for 10 seconds and you will not have a barrel as hot as one shot 3 times in the same time period...
exposure to 2000 deg temps for extremely short periods cannot adequately explain, in my mind, the process of barrel heating...

friction seems like a much more likely source...
and mathman is correct... friction can convert mechanical energy into heat... and in very short order...

FWIW, here's another easy-to-see example of the crucial distinction between the nature of heat (�F) and the volume of heat (Btu) �

� two birthday cakes � one for a one-year-old, the other for any eighty-year-old �
� one with one candle, one with eighty candles (all candles, on both cakes, as nearly identical as possible)
� Every candle burns at the same flame temperature (�F).
� The air above the eighty-candle cake is eighty times as hot (Btu) as the air above the one-candle cake.

I've had several cheap electric heaters that got very hot (�F) and would put-out only 1,500 Btu.

I have one now that doesn't get as hot (lower �F) but puts-out 2,300 Btu.

(Different technologies)