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

while the gas laws are for mostly ideal conditions, they none the less give a good start. No math model will include everything, but each different one can give a different picture for understanding. Just using the ideal, work, and a little calculus will give some pretty good measurements. Taking the area of the bore x pressure (converted) to get the force, then integrating for work, then conversion to velocity will account for abut 98-99% of the work done in the system when compared against the readings when chronoed. Again using the pv equation and using the volume of the bore the muzzle pressure and exit velocity can be caluculated. In any event the curve is almost flat after the first 12 inches, indicating little acceleration (almost constant velocity) and obviously because its always hotter towards the chamber. The amt. of work engraving the bullet from the rifling's far exceeds the heat generated by friction. Not only does the zinc in the gilding metal act as a lubricant, but the metal and the residue from the last firing, along with small amts. of oil that were polymerized to varnishes and lacquers. There is little doubt friction causes heat build up, look at your auto engine, but the coefficient of friction for the bullet when compared to the other forces can be ignored to linearize the solution.
As far as the question the op asks, the barrel heat increase between a 204 and a swift shooting the same velocity and pressure is do to the increase in propellant wt. alone.

The numbers just don't add up. The highest reported bullet drag force I've seen was in an Army report claiming 400 lb for a .30 conventional bullet (original X bullets were much higher). Over a 2 ft barrel, that's 800 ft-lb total drag energy which works out to 1 BTU.

A 3 lb barrel needs about .36 BTU for each 1 F of temperature, so we're looking at a 3 F rise from bullet drag, and that's the worst case numbers I could find--and it assumes all the friction heat goes to the barrel with none to the bullet. Other sources give a far lower average bullet drag force.

The 7.62 NATO releases about 11 BTU of chemical energy. Hatcher's notebook has some old military data suggesting 22% of the gas energy goes to barrel heating and 7% to bullet friction. Now, 7% of 11 BTU is .77 BTU, about in line with the 1 BTU figured above.

My notes don't record if Hatcher's 22% for barrel heating includes or excludes the 7% for bullet friction. The table I recorded suggests it is excluded, so his numbers imply convective and radiative heat transfer from the gases to the barrel are triple the heat energy of bullet friction.

Does it really matter? Personally I think it is more from burning powder than friction/velocity. This is based on nothing but opinion. I do know that a .257 Weatherby will get hot quicker than a .257 Roberts, and I can't see where the friction would be any different. The amount of powder burned is considerably different.

Etoh,

I don't think your post was present when I started mine, but it was there after I clicked SUBMIT. You did answer the question, I just didn't see it in time. Thanks for the insights.

Hi folks... great thread!

Just wondering if anyone caught this. This is from an article on Berger bullets written by Berger. It's in reference to how very occasionally, one of their thin-jacketed VLD's would literally blow up midair...

"As it turns out the bullets were heating up to the point where the cores would actually melt. Once a bullet leaves the barrel with a melted core it is certain that the molten lead will burst through the jacket under such high RPM. Obviously this was a problem that we needed to resolve so we decided to test a thicker jacket. Making the jacket thicker did not make it strong enough to contain molten lead but rather it moved the lead away from the source of the heat. The source of the heat that can melt a core is the friction between the bearing surface and the rifling as the bullet travels through the barrel."

http://blog.sinclairintl.com/2011/03/16/history-of-the-match-grade-berger-hunting-vld/

this aspect of the topic is about 30 yrs. old, and was frequently asked to speer enj. about their varmint bullets in swifts which seem to vaporize out the barrel. There answer was that the engraving from the rifling was cutting through and the bullet lost its jacket. While the number of revs may be a first daunting it still has to travel a foot or so to complete one revolution, to in say 400 yds its only completing 1200 rev.-- not that much. Softer lead will obtruate more than harder lead giving the appearance of being molten, as any bullet caster knows. If this is the complete story it will be easy resolved by contacting Brian Litz at Berger for the rest of the story.

Etoh,

The promlem is not the number of revs, but the rate. If the twist rate is 1:12", and velocity is 4000 fps, then the rotation is theoretically 240,000 rpm. That sounds pretty fast for a thin copper jacket containing [molten] lead. I've seen some things blow-up at much lower rotation rates than that.

Sure but its not spinning long enough for that force to act on it. Maybe 1/2 second. 240000/60= 4000/2=2000revs per complete time of flight cycle.

all i will say is even with alot of shooting ive never had a pellet or bb guns barrel get to hot to touch.......

I've never had a bullet blow-up in mid-air, but in the instances of such that I have read about and heard about, it happened pretty close to the barrel's muzzle, not 2000 feet downrange.

"I've never had a bullet blow-up in mid-air,..."

I have. It was those Speer Plinkers of about 100 grains in .30 cal. I loaded some to about 3000 FPS in a .30-06, and they blew up about 10-15 feet from the muzzle.

I don't know if these bullets are still made or not. They were light bullets with a lead core and a half jacket.

I have also seen .220 Swift bullets leave a vapor trail, or else maybe it was a tail of melted lead that I saw.

I've had a number of bullets come apart after leaving the muzzle in various degrees.

Sometimes they come totally apart soon after exiting. If this happens a dark cloud will be seen briefly, more often by somebody else than the shooter. Though I have seen it through the scope. It's particularly interesting when shooting at a prairie dog on a typical dry summer day, and no dust cloud shows up anywhere near the prairie dog--which of course survives.

I've also seen a lead swirly-pattern around bullet holes on a paper target, and also seen bullets zip way off to the side of where they were aimed--as in as foot or two instead of a couple of inches.

It's possible to put some real numbers to the question.

From my previous post, the work that is converted to heat is 300 ft-lbs, more or less. That is the energy that is available as heat, and it is much less than the energy originally contained in the powder. So most barrel heat still comes from the original heat of the powder, not the derived heat of friction. However:

300 ft-lbs is .4 KJoules.

A 180 grain bullet (arbitrary choice) has .0116 Kgrams of mass.

It requires 23 KJoules of heat per Kgram to melt lead.

.4 KJoules/.0116 Kgrams = 36 KJoules/Kgram of available heat from friction. This is greater than the 23 Kjoules/Kgram required to melt lead. But for this to happen, most of the heat from friction would have to end up in the bullet. I don't think that is a reasonable assumption because a lot of the friction heat ends up in the barrel. So I don't think that the heat of friction would come close to melting a 30 cal 180 grain bullet. Other bullets require a fresh calculation. I would expect that if bullets are to melt from friction, they would have to be smaller, lighter bullets.

Sierra contains or used to contain a warning with 22 cal blitz not to shoot over 3500 fps. because of separation.

Denton:

What about the conductivity of the mostly copper jacket? Copper conducts heat much better than steel, so it seems likely that most heat from friction, if any, would be conducted through the jacket and into the lead core.

I have fired high velocity bullets into dirt banks and dug them out soon after, and many were too hot to hold in my hand. The heat came from somewhere, either from friction or the powder gases.

I don't think it is too unreasonable to think that some of the heat comes from frition.

As an example, take the SR 71 airplane. I have read that some parts of this plane get red hot at speeds approaching 2000 MPH, and this is because of friction from the air, and thin air at that, because of the altitude.

Many bullets are traveling faster than 2000 MPH. Is it unreasonable to think that friction from the bullet's flight through the atmosphere would not produce heat?

This is no argument. I find this to be an interesting subject, and I would like to learn more about it.

1--7: To answer only part of your question about hot bullets retrieved from dirt.

Those bullets ALSO came under intense friction upon contacting the dirt or whatever the medium. However warm they may have been from leaving the barrel, they were heated much more upon contact with the dirt.

If you ever shot a bullet into SAND, it's like being exposed to a sand blaster or belt sander. IF there's anything left it's HOT too.

IMO the HOT brass cases ejected from semi or auto rifles PROVES the majority of heat is produced by the BURNING POWDER. Those cases have NEARLY no friction force upon them for any length of time or distance traveled.

MD,Mr. Howell, & Denton gave the essence to the question on P 3-4 of this thread. I concur.

JWALL
__________________

VEGETARIAN.........Indian Word For Poor Hunter

usually the pressure or shock wave ahead of the bullet protects the bullet from rain, sleet, and hail, air. Some effects from spin drift, earth spinning, the need to convert to knots when shooting over water etc. The biggest effect comes from gravity. The time of contact of bullet with barrel wouldn't allow for that much heat content to transfer.

The vapor tail is called a trace by the way, and there are some really good downloads on sniperhide if you wish more on that.

Interesting questions.

I've noticed three things that cause these rifle barrels to get hot and hotter:
1. Chemical reaction in there, one product of which is heat.
2. Some friction (bullet to bore) which will cause a bit of heat.
3. Sunshine on the barrel on a clear/warm day.

The speed (quicker?) at which the barrel gets hotter seems affected by several factors: quantity of heat (chemical + physical)) generated; frequency of applied heat (time between chemical reactions - shots taken); the propensity of that barrel to have its accumulated heat dissipate; ambient temperature and wind (heat exchange factors) and maybe other minor factors.

I don't think you're thinking about this quite right. ;-)

From the instant the bullet leaves the barrel (using your figures above), the widest portions of the jacket are enduring over 180,000 G of centripetal force (if my calculation is correct, but I'm pretty sure it is...it's just r*w*w where w is the angular velocity in radians / sec - I converted everything to MKS units). If you were to endure 180,000 G in a floorward direction for 1/10 of a second, you'd be a thin red film on the floor of whatever you happened to be standing on. Hitting concrete at terminal velocity would be much less traumatic.

That is a LOT of force working to rip the jacket off the core. I've always been amazed bullets resist deformation as well as they do.