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

I have a question concerning a barrel getting hotter as it is shot numerous times:

Is it higher velocity that makes a barrel heat up quicker, or is it higher pressure that makes the heat?

I realize that they are almost directly correlated, but am wondering about using reduced loads to lower barrel heat while prairie dog shooting. I can reduce the pressure of a known load to get lower velocity, or I can use a different faster powder that creates continued high pressures but lowers the velocity. Which one will give me less heat?

My opinion is that it is the friction of the bullet passing through the barrel that causes the heat.

There might be some heat generated by the hot powder gasses, but I think most of comes from the bullet's travel through the barrel. Lower velocity would probably prevent the barrel from heating up as quickly as high velocity.

But, I don't see where a slow or fast powder would make all that much of a difference, if the velocity stayed constant.

You don't necessarily want to reduce a slow burning powder to get low velocities. Consult a loading manual and use a powder suitable for the velocities you are looking for. Also be careful and don't use too much of the fast powder.

All the heat comes from the powder. No question about it. It's as simple as that.

The friction drains energy � it does not produce it.

The powder � slightly assisted by the primer � is the sole source of energy in a nonrocket cartridge. Several things drain it (friction, recoil, noise, etc) leaving only about 25�30% propelling the bullet.

Where else would cartridge energy come from?

Let a hot summer sun heat an idle rifle with a loaded round in the chamber, and it'll still be the heated powder that expends its energy when you pull the trigger.

We talk about (and measure) pressures, but it's the heat that's associated with those pressures that makes the barrel hot.

If you want to amuse yourself with intuiting some phenomena of interior ballistics, tune your noodle to this �

A pressure-cooker in your kitchen can produce some down-right nasty havoc if for some reason it lets go at or near its normal operating pressure of 20�30 lb/sq in. Try comparing that with a high-velocity cartridge operating at something like 60,000 lb/sq in.

That's interesting as hell. I would have thought the friction caused the heat as well.

Friction can certainly convert mechanical energy to heat.

I think it is both of the heat produced by combustion of the powder and the friction of the bullet moving down the barrel. Combustion of the powder produces both heat and kinetic engery of the moving bullet and gases. The friction in the barrel converts some of the kinetic energy to heat. What the balance is between these two sources of heat is I have no idea.



Whether using a bullet with a short bearing surface and mild charges of a faster burning powder would make much of practical difference I couldn't say.

Ken is correct.

Pressure and heat are manifestations of the same microscopic thing: molecules in motion. The hotter they are, the faster they move and the more force they exert on their surroundings.

Consider that propellant gas peaks at 2,000 degrees or more, and I think you'll soon find where the barrel heat comes from. It's the same mechanism that makes a car engine hot.

Friction contributes little to barrel temperature.

I've actually shot coated XLC's and uncoated cup and core bullets from the same rifle (223WSSM), at the same velocity and measured the heat. The coated bullets produced far less heating on the barrel. From memory, three naked bullets produced about the same heat as five coated bullets.

Friction would seem to be the likely difference. FWIW, Dutch.

I wasn't arguing the percentage contribution. But Ken's statement wasn't clear as to how friction was stealing the powder's energy.

The force of friction in a barrel is around 150 pounds, more or less. Working over 2 feet of barrel, that's about 300 foot pounds.

In a 30-06, that's not the dominant source. In a 223, it's a greater proportion of the total but still not dominant.

OK. Let's look at that. (I wasn't trying to explain the obvious � just stating the facts, Ma'am.)

� The powder burns, converting latent energy into operating energy.

� That energy pushes the bullet forward.

� The moving bullet rubs against the sides of the bore.

� That rubbing � friction � produces heat.

� The barrel absorbs some of that heat (and the bullet gets some of it) as well as the greater heat that the combustion produces.

� The push that produces the movement that produces the rubbing produces some of the heat that the barrel absorbs.

� The heat that the barrel absorbs does not push the bullet.

� The heat that the barrel absorbs stays behind while the bullet goes wherever it's going.

� The heat is one expenditure of the powder's energy. The push is another. So not all the powder's energy is spent as push.


X-Y=<X.

Now please don't ask me to explicate the fundamental distinctions between British thermal units (Btu) and degrees Fahrenheit (�F)!

denton's number for bullet friction sounds familiar. Going through the math, you get a temperature rise of about 1 F from the bullet's friction. The rest of -- the bulk of -- the barrel heating comes from the gases. The heat transfer comes from convection (molecules banging on the metal) and radiation (light absorbed by the metal).

To reduce the temperature of the gases, reduce the pressure.

Coated bullets need less pressure to make the same velocity, so the gas temperature will be lower.

Stretch a rubber balloon a bunch of times and feel it... bend wire back and forth repeatedly in the same spot, and then feel it.

Barrels also swell slightly in an advancing wave trailing the bullet. There is a bit of internal friction there which is still sourced from the burning powder's energy, but not due to bullet/barrel friction.

The venting gas, post exit, cools significantly (Joule-Thomson effect) through expansion and stays in the barrel far longer than bullet transit time.

2525
Suspect you used convection where you meant conduction...
art

The latent energy of smokeless powder is, IIRC, about 175�180 foot-pounds per grain. If all the energy from fifty grains at 175 ft-lb/gr were expended on propulsion alone, the kinetic energy of the bullet would be between 8,000 and 9,000 ft-lb (8,750, according to my creaky math, bleary eye, and dull pencil).

(IIRC, the figure is about 178 ft-lb/gr for the IMR powders. But I'm 'way too tard and sleepy to look it up.)

Ken, I hope you're simply tir'd and not actually going "tard" at this point.

So bottom line is: if I reduce the pressure, it will thereby reduce the heat going into the barrel?

Yessir. This is why hot loads lead to shortened barrel life

Yep. PV=NRT. All things equal higher pressure is directly correlated to higher temps.

Bullet friction heating is immediate--whatever the amount, it dumps the heat right into the barrel steel. Hot gasses heat via convection and radiation. Radiation is fast, whereas convection is slow.

Radiation heat transfer depends greatly on the temp., and given the very high temp. and the very short times involved, it's almost certain the hot gas heats via radiation.

It can also be argued that friction frequently decreases with higher rubbing speeds, so my vote is barrel heating is largely via radiation from the hot gas.

Too bad we can't include jpegs in the posts. I have some rather interesting photos of just how fast pure radiation can heat heavy steel cables up and vaporize them in well under a thousandth of a second.

Pour some powder out on the ground. Light it with a match and hold your hand over it.Then take a bullet and rub it like hell on a piece of steel and hold that steel with your hand.
See which burns you the most.