Rifle barrels getting hot??

**Rheghead** · 31-Oct-09, 23:53

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?

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.

**Rheghead** · 01-Nov-09, 00:24

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

**Stavro** · 01-Nov-09, 00:30

Originally Posted by Rheghead

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?

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

, thought provoking indeed. I'd better get off to bed before I go ballistic.

**Phill** · 01-Nov-09, 00:37

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.

**Stavro** · 01-Nov-09, 00:41

Originally Posted by Phill

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.

**M R** · 01-Nov-09, 01:13

Originally Posted by Stavro

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

**Rheghead** · 01-Nov-09, 01:46

Originally Posted by Stavro

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.

**Stavro** · 01-Nov-09, 02:10

Originally Posted by Rheghead

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.

**Stavro** · 01-Nov-09, 02:13

Originally Posted by M R

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.

**Rheghead** · 01-Nov-09, 02:19

Originally Posted by Stavro

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?

It's getting late and my brain is in a spin trying to figure it out.

Where is northerner when you want him?

**Stavro** · 01-Nov-09, 02:25

Originally Posted by Rheghead

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?

It's getting late and my brain is in a spin trying to figure it out.

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

Originally Posted by Stavro

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.

**Rheghead** · 01-Nov-09, 02:51

Originally Posted by Stavro

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.

**Stavro** · 01-Nov-09, 03:35

Originally Posted by Mystical Potato Head

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

, but time for bed.

**M R** · 01-Nov-09, 03:51

Originally Posted by Stavro

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

::

**Phill** · 01-Nov-09, 09:10

Originally Posted by Rheghead

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?

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?