What does a hot barrel really do?

[Vairochana]

Gday, I was sitting there thinking about my .303 as is my want, in particular about the strange groups I get with military ammo.
I was thinking that my barrel heats up fairly quikly with the military stuff and that led me to wonder exactly why barrels act the way the do when they get too warm.
Now I sat there trying to remember high school and undergrad physics, alas I have no technical metalurgical knowledge.

I thought:
Does the barrel expand and the rifling not work efficiently?
Does the diameter skink and the bore become tight?
Does the barrel become ininitesimally more or less flexible and disturb the harmonics?

Anyway i got confused and decided I needed a beer.
So if there are any experts out there or anyone wants to have a stab at it-I am all ears (no complex maths if possible please)
Cheers

 

[mete]

Expansion is not the problem though exapnsion from heat would make the bore and outside of the barrel larger. If the barrel has stresses in it the heat tends to make the barrel bend a bit giving you a different point of impact. When it cools down the barrel goes back to the original POI. Not a problem for a hunting rifle where 2 or 3 rounds may be fired but is a problem in a target rifle.

 

[Radjxf]

I'd guess you're seeing the effects of expansion and contraction ultimately in terms of what it does to the barrel's harmonics. How the barrel "whips" in relation to the stock (vibrations, harmonics) changes when a barrel heats up. I find this really noticeable on little 17HMR rifles, especially the cheapies with poorly centered crowns.

 

[Firehand]

Good question: since the steel of the barrel expands, would the bore become slightly larger or slightly smaller as the barrel heats?

On thing that does happen is that if the barrel is under uneven stresses, either from touching the stock or uneven machining, that affects how it vibrates and can actually cause it to flex in a particular direction. Interesting combinations of things happen.

 

[orangelo]

There is a picture of a M2HB in last month's Small Arms Review that was fired until failure. The barrel heated up until it started drooping and exploded and the bullet came out the side. That can't be good for accuracy.

 

[BIGDADDYLONGSTROKE]

Just curious, but I wonder abour how many rounds could be fired from an Ar-15 say a bushmaster before the barrel started drooping from the heat?

 

[carnaby]

Good question: since the steel of the barrel expands, would the bore become slightly larger or slightly smaller as the barrel heats?

Both the outside of the barrel and the bore expand due to increase in temperature. We did a little experiment in high school where a brass ring that just barely prevents a bearing ball from passing through is then heated up. After that, the ball passes through easily. It should be a simple matter to compute the thermal expansion of the bore.

 

[carnaby]

OK, I whipped up the calculation and it seems that a .30 cal barrel that goes from 75F to 200F will go from .308" to .3083". If my calculations are correct, it doesn't seem like 3/10's of a thou are going to make that much difference in terms of bullet/bore interaction. Internal stresses in the barrel and how they act under changes in temperature seem like the more significant factor in changes in accuracy as the barrel warms up.

Now, how hot does the barrel get? I just guessed at 200F.

 

[XDKingslayer]

Barrels can get pretty hot depending on their use. They don't get too bad when we use them like we do for target practice or training, but military use they can get pretty hot.

One poster above outlined a .50 barrel getting hot enough to bend, which it will do. I've had mine hot enough to glow red and you would swear you could see the rounds going through it.

If you want a good demostration on what a hot barrel can do accuracy wise, get a Winchester Model 94 hot and watch it throw rounds everywhere.

 

[sparx]

If the barrel has stresses in it the heat tends to make the barrel bend a bit giving you a different point of impact. When it cools down the barrel goes back to the original POI. Not a problem for a hunting rifle where 2 or 3 rounds may be fired but is a problem in a target rifle.

+1. When sighting in a nice high-caliber rifle it's best to let the barrel cool down after three shots.

 

[Zak Smith]

It has to due with rifle-build quality and barrel quality, and the order of operations when the barrel was cut. Stress left in the steel, or in the action/barrel joint, can cause warping when the barrel is hot.

I regularly shoot my 308 AI-AW until it is too hot to grab the barrel, and neither the POI nor group size is really affected.

 

[Firehand]

Thanks, Carnaby. I'd seen that experiment before, but forgot about it.

I shouldn't try thinking in the morning...

 

[30Cal]

Residual stresses, manufacturing flaws (off-center bore) and interaction with the stock (especially with a full length military stock) are the things that will cause a rifle to shift as it heats up.

A centerfire match rifle is built to take it. If it shifts when the barrel heats up, then it's a poor match rifle.

25rds in 50 seconds, iron sights, 600yds, match AR

 

[BigFatKen]

If you fire an M-60 in .308 for too long which depends on how old it is, your tracers start to give you "spinners".

 

[pcf]

A hot barrel will burn your finger. Ouch!
A hot barrel will melt you gun case, when you set your rifle on it. Oops.
If there's cosmoline on your hot barrel, it'll start to smoke. Stinky.