Rifle barrels getting hot??

[Gronnuck]

A barrel of any weapon has to fire an awful lot of rounds in quick succession before it becomes inoperable. I'm no armourer but it was explained to me that the rifling inside the barrel not only spun the round but assisted/allowed for expansion.
Auld sweats will remember carrying a spare barrel if they were part of the LMG/GPMG team.
Artillery also had the facility to swap barrels. I believe a 105mm howitzer barrel could be swapped in something like 12 minutes.

[compo]

aye it was allways a concern with the general purpose machinegun that due to overheating there could be a breach exsplosion so to try to counter that the barrrel should be changed every 200 rounds. but rate of fire and urgency would determine if you could afford to stop and change out the barrel. i have seen them so hot at night you could actualy see the bullets going up the barrel. as for sniper rifles or precision target shooting its best to fire from a cold barrel as the barrel is free floating and exspansion could cause the barrel to ceas to be free floating thus affecting the acuracy. well so i have been led to believe.

[Rheghead]

So can we clarify? When the barrel get hot and expands does the bore tend to get bigger or smaller?

[Stavro]

Well the way I'm looking at it is that the heat restraining substance needs to be on the outside of the barrel thus limiting the effects of expansion and bore growth under heating conditions, so that bullet accuracy is maintained over a wider range of temperatures.

I was thinking about this problem this morning and it seemed to me that it is similar in a way to the modern method of welding rail track together. This caters for thermal expansion and contraction by having the rail under tension. So I did a search and found several articles describing such prestressing in rifle barrels. For example:

"The earliest method of applying this principle was to heat steel ring-shaped jackets, or hoops, to high temperatures, then slip them over the gun tube and allow them to cool.

"As the hoops cooled, they contracted, until at the end of the process they were squeezing the gun tube inside with a pressure of thousands of pounds per square inch.
Guns so constructed are known as built-up guns, and are still made in sizes over 8-inch.

"About the time of World War I, the same principle was applied to monoblock (one-piece) guns in the radial-expansion or auto frettage process.

"In this process, a single steel gun tube whose bore is slightly smaller than the caliber desired is filled with hydraulic fluid.

"The pressure is then built up enough to enlarge the bore permanently about 6 percent.

"When the pressure is released, the outer layers of the tube tend to return nearly to their original dimensions, while the inner layers, which have been considerably enlarged, tend to maintain their increased diameter.

"The result is that the inner layers of metal are severely compressed by the contracting force of the outer layers, just as if a hoop or jacket had been shrunk on. In other words, the tube is 'self-hooped.' ”

This was from http://www.eugeneleeslover.com/USNAV...RUCTION-1.html

Under such extreme tension, the inner metal tube would thermally expand so as to produce a slightly smaller bore. This seems to fit all the evidence, I hope!

[Tristan]

The way I see it is this, when I did my apprenticeship in Engineering, if I wanted to slip a tube over a shaft that has an interference fit, I needed to heat the tube to get it over the shaft. So I applied the same engineering principle with a barrel (tube) and the bullet (shaft), as the barrel gets hotter the bullet is slack. But my learned firearms expert friend says different it get tighter on the round, so when I took his word for it, I still have grave doubts and he is rarely if ever wrong on such like. It may have something to do with the bimetal make-up of the barrel expanding at different ratess

More confused

What was the thickness of the tube? It will expand in all three directions but the thin tub will be pulled wider
A thicker gun barrel will expand in all three directions making the bore smaller

[Rheghead]

That was about barrel construction and processes associated with that. I'm on about when a barrel heats up during a firefight or training exercise, does the bore get bigger or smaller?

[Rheghead]

What was the thickness of the tube? It will expand in all three directions but the thin tub will be pulled wider
A thicker gun barrel will expand in all three directions making the bore smaller

Yeah but the bigger tube would get hotter as well thus radially expand as well thus the liner internal diameter will enlarge to suit as well, no?

Obviously, the expansion in all three dimensions is related to the length in that direction. The circumference is longer than the thickness so the barrel should expand I would have thought.

[sids]

It's not something you can solve by debate. No-one here appears to know.

Have you tried Google?

[Rheghead]

It's not something you can solve by debate. No-one here appears to know.

Have you tried Google?

Very true, I've tried google and I'm not that good at googling but all early indications support that the bore gets bigger thus supports my theory but no explanation why.

[northener]

I had a quick ferret around and found this thread with a lot of ex-Forces discussing the same issues - but with regards to cold temperatures. It should point us in the right direction:

http://www.militaryphotos.net/forums....php?p=4506916

BTW, if a barrel heats up - the internal diameter increases...old bronze artillery pieces (which expand at lower temperatures than iron/steel) had to be cooled down with wet cloths being draped over the barrels, as when they overheated the accuracy went out of the window.

[sids]

How about this?:

For the bore to get smaller, the metal around the bore would have to be compressed, but steel doesn't compress easily.

[Rheghead]

I think I've found the answer that I'm looking for on this webpage.

http://www.patentstorm.us/patents/46...scription.html

The expansion of barrel dimensions due to thermal excursions in present and prior art alloys prevents long burst firing because as the bore of the gun expands the projectile does not interact desirably with the rifling in the bore and does notattain the proper spin or trajectory. Trajectile accuracy for barrels operating with higher projectile firing rates and at higher propellant energies will be an important criterion nevertheless

So the barrel bore does get bigger with temperature, I'll pass that on to my pal.

Thanks for your help and interest!

[compo]

yes it does exspand

[George Brims]

From a physicist turned mechanical engineer (me): As the barrel gets hotter, all of its dimensions will get bigger, and that includes the inside bore, which will get bigger.

This applies to any cylindrical piece of metal. That's why there is for instance a thing called a bearing heater, which is a cone-shaped heater used to heat a ball bearing so it will slip onto a shaft.

The reason a barrel will warp when it gets hot is that it's cooling to the air as it's gathering heat from the rounds being fired, but where the grip underneath is covering it, it can't cool as quickly, so it isn't at a uniform temperature.

[Whitewater]

Followed this thread through to the end, I was having a quiet chuckle to myself. When you heat up a tube or ring the bore gets larger. Simple principle used in engineering and many other trades for many years, steel hoops going round wagon wheels are heated to get them on, steel sleeves fitted around tubes or solid bars are always warmed before fitting. Same thing applies to rifle barrels when warmed up, thats why they have to be cooled or changed in order for the bullet or shell to maintain its accuracy. If you just think about it, it would be pretty dangerous, in fact deadly, if a bullet/shell was leaving the breech and tried to enter a barrel that was too tight, it would probably blow your head off.

[Stavro]

If you just think about it, it would be pretty dangerous, in fact deadly, if a bullet/shell was leaving the breech and tried to enter a barrel that was too tight, it would probably blow your head off.

The temperature changes are such that the expansion/contraction would be microscopic. Not enough to cause an explosion nor enough to significantly alter accuracy, depending on whether the round gets tighter or slacker.

[George Brims]

You're not going to get an explosion because the barrel heats up, but you may lose accuracy if as I noted above the barrel is not able to shed the heat uniformly, so there's warping.

On the other hand, if you're creeping along behind a bank preparing to assassinate some waterfowl, and get mud in the muzzle of your shotgun, you will get an explosion when you pull the trigger.

[Stavro]

When metal is prestressed and thus under high tension, the effect of temperature over a limited range is to increase or decrease that tension. Hence rail track is welded together at the ends, rather than having gaps as before. The track does not buckle under fluctuations in temperature.

The rifle barrel is also prestressed on the outside, not the inside. The inside expands with increasing temperature and has to push against the force produced by the tension on the outside layers of metal. Under these circumstances, the inside of the barrel may not act like a heated bearing (or jam pot lid), but would tend to expand such as to reduce the bore slightly.

Our learned friend was stating that if the bullet was too big for the bore, then the shooter would risk serious injury or death. This serious injury or death, in this scenario, would be caused by the significant reduction in bore size that was implied and the rapidly expanding gas from the explosion ripping through the gun near the shooter's face.

Nothing to do with the murdering of ducks.

[northener]

http://www.argospress.com/jbt/Volume8/8-3-1.pdf

Mentions overheating and thermal distortion (warping) at the end of para. 3.