View Full Version : Rifle barrels getting hot??
This should appeal to our ex-servicemen and metallurgy experts, when a rifle has been fired numerous times and the barrel gets hotter and expands, does the barrel get tighter on successive rounds or get slacker?:confused
I thought slacker but got told different tonight which seemed counter-intuitive to me so not entirely sure. Thanks in advance.
redeyedtreefrog
01-Nov-09, 00:05
I'm no expert but i'd think, seeing as the metal is expanding, it would squeeze inwards as well as growing outwards- It would expand in all directions, not just outward?
Metalattakk
01-Nov-09, 00:05
Think about it - the metal that makes up the barrel expands with the heat, not the empty hole. ;)
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 rates.
More confused :~(
This should appeal to our ex-servicemen and metallurgy experts, when a rifle has been fired numerous times and the barrel gets hotter and expands, does the barrel get tighter on successive rounds or get slacker?:confused
I thought slacker but got told different tonight which seemed counter-intuitive to me so not entirely sure. Thanks in advance.
The metal of the barrel will expand in all three dimensions according to its coefficient of linear expansion. This means that the thickness of the barrel side wall will increase both inside and out with temperature and the round will then encounter a tighter (smaller) bore.
Kevin Milkins
01-Nov-09, 00:36
Mm :roll:, thought provoking indeed. I'd better get off to bed before I go ballistic.
Not simples me thinks. Depends on the makeup of the barrel.
I understand the problem of expansion/contraction is both! The temperature isn't uniform along the length so actually warps the barrel.
The only thing I really know is don't touch the barrel after straffing a few rounds off.
Not simples me thinks. Depends on the makeup of the barrel.
I understand the problem of expansion/contraction is both! The temperature isn't uniform along the length so actually warps the barrel.
All metals expand with temperature. The effect is more pronounced near the stock, but the barrel should not warp along its length.
All metals expand with temperature. The effect is more pronounced near the stock, but the barrel should not warp along its length.
Barrels will warp.. Pretty sure the have to be changed out fairly often, i guess to enure saftey and good hits on target. Shorts bursts are fine fine very short periods, sustained bursts of fire = extremete heat = warped barrel = change it, or could be like a bad firework show
The metal of the barrel will expand in all three dimensions according to its coefficient of linear expansion. This means that the thickness of the barrel side wall will increase both inside and out with temperature and the round will then encounter a tighter (smaller) bore.
I understand with what you're saying but I've got a problem with it.
As a metal heats up the atoms start to move or vibrate more thus occupying a greater volume. The trouble with what your saying is that for the internal diameter to get smaller, the atoms on the inside surface need to occupy a smaller volume which when hot is counter intuitive to me because I'm certain that the forces to counteract the expansion force is collossal. I think the hole gets larger because all the atoms are hotter so all atoms are further apart.
I understand with what you're saying but I've got a problem with it.
As a metal heats up the atoms start to move or vibrate more thus occupying a greater volume. The trouble with what your saying is that for the internal diameter to get smaller, the atoms on the inside surface need to occupy a smaller volume which when hot is counter intuitive to me because I'm certain that the forces to counteract the expansion force is collossal. I think the hole gets larger because all the atoms are hotter so all atoms are further apart.
Hi Rheghead. I think you are saying the same as I am, because the expanding metal will take the path of least resistance and will thus expand into the air inside the barrel rather than trying to accommodate itself in the metal in the opposite direction. Thus the bore would become smaller.
Barrels will warp.. Pretty sure the have to be changed out fairly often, i guess to enure saftey and good hits on target. Shorts bursts are fine fine very short periods, sustained bursts of fire = extremete heat = warped barrel = change it, or could be like a bad firework show
I would agree with you except for the "extreme heat" part. They might become hot with respect to our skin temperature, but compared with what a metal will withstand, they do not usually suffer "extreme" heat. They will thus not warp to any significant degree.
Hi Rheghead. I think you are saying the same as I am, because the expanding metal will take the path of least resistance and will thus expand into the air inside the barrel rather than trying to accommodate itself in the metal in the opposite direction. Thus the bore would become smaller.
So surely the path of least resistance must be one which does not involve crushing metal atoms closer together under heating? So the only route for expansion would be the bore size to get bigger?:confused
It's getting late and my brain is in a spin trying to figure it out.[lol] Where is northerner when you want him?
So surely the path of least resistance must be one which does not involve crushing metal atoms closer together under heating? So the only route for expansion would be the bore size to get bigger?:confused
It's getting late and my brain is in a spin trying to figure it out.[lol] Where is northerner when you want him?
No, that would only be true if there were some restraining substance inside the barrel, but even then it would have to be enormously strong. Consider the force that water exerts when tranforming into ice - enough to split huge rocks.
Mystical Potato Head
01-Nov-09, 02:44
Well if the bore gets smaller with the heat why on numerous occassions have i heated a bearing so that it expands to fit a shaft and then cools to give an interference fit,giving you one rather tight fitting bearing?
I'm curious too Rheghead.
Metalattakk
01-Nov-09, 02:45
No, that would only be true if there were some restraining substance inside the barrel, but even then it would have to be enormously strong. Consider the force that water exerts when tranforming into ice - enough to split huge rocks.
Indeed. It would be a completely different story if the end of the gun barrel was closed, for instance. ;)
No, that would only be true if there were some restraining substance inside the barrel, but even then it would have to be enormously strong. Consider the force that water exerts when tranforming into ice - enough to split huge rocks.
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. :confused
Well if the bore gets smaller with the heat why on numerous occassions have i heated a bearing so that it expands to fit a shaft and then cools to give an interference fit,giving you one rather tight fitting bearing?
I'm curious too Rheghead.
Yes, this is certainly true, a bit like running a jam lid under hot water to loosen the lid. In this case, Rheghead is correct and the restraining layer would need to be on the outside.
An interesting question :eek:, but time for bed. :D
I would agree with you except for the "extreme heat" part. They might become hot with respect to our skin temperature, but compared with what a metal will withstand, they do not usually suffer "extreme" heat. They will thus not warp to any significant degree.
but they will warp to the point that the user of that said weapon, will ask for a replacement for the said barrel, ::: i i sir, tis time for the barrel to be changed is a common fraze used on the battle field :D ::
This should appeal to our ex-servicemen and metallurgy experts, when a rifle has been fired numerous times and the barrel gets hotter and expands, does the barrel get tighter on successive rounds or get slacker?:confused
I thought slacker but got told different tonight which seemed counter-intuitive to me so not entirely sure. Thanks in advance.
Why the concern anyhow?
Are you considering a Silver bullet shooting frenzy in Portgower?
:eek:
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.
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.
So can we clarify? When the barrel get hot and expands does the bore tend to get bigger or smaller? :confused
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. :confused
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/USNAVY/GUN-BARL-CONSTRUCTION-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! :eek:
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
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?
: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
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.
It's not something you can solve by debate. No-one here appears to know.
Have you tried Google?
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
01-Nov-09, 17:00
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/showthread.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.
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.
I think I've found the answer that I'm looking for on this webpage.
http://www.patentstorm.us/patents/4669212/description.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.:evil
Thanks for your help and interest! :Razz
George Brims
03-Nov-09, 00:30
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
03-Nov-09, 00:47
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.
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
03-Nov-09, 02:21
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.
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
03-Nov-09, 11:11
http://www.argospress.com/jbt/Volume8/8-3-1.pdf
Mentions overheating and thermal distortion (warping) at the end of para. 3.
Powered by vBulletin® Copyright © 2024 vBulletin Solutions, Inc. All rights reserved.