Rifle barrel expansion with increasing temperature

[ezee]

Question for any engineers or those in the know, does the bore diameter of a rifle barrel increase or decrease as its temperature increases and the metal expands? Friends and I today trying to explain the drop in muzzle velocity of cartridge (same load, same rifle) of 150 fps from 90 to 35 degrees. Cartridge temps approx the same in both instances with heat stable powder. Is there a significant change in barrel resistance that could account for slowing in muzzle velocity with decreasing temperature with other variables being held constant?

 

[doubled]

Re: Rifle barrel expansion with increasing temperature

it is going to depend on how sensitive the load’s powder is to air temperature....

 

[ezee]

Re: Rifle barrel expansion with increasing temperature

Got that. As stated above cartridge(powder) temps about the same in both instances room temp say 70 degrees.

 

[6brshooter]

Re: Rifle barrel expansion with increasing temperature

Same lot of powder? Same lot of primers? Was barrel clean or dirty this time compared to last chrono session?

 

[ezee]

Re: Rifle barrel expansion with increasing temperature

Same lot powder,primers,brass,fouled barrel both instances. Just curious if bore expansion/ contracture with temperature change could make a significant difference in resistance in bullet/ barrel interface causing a change in muzzle velocity. Deep I know.

 

[Force_Multiplier]

Re: Rifle barrel expansion with increasing temperature

ammo heating up in the chamber would probably increase muzzle velocity more

 

[Casey Simpson]

Re: Rifle barrel expansion with increasing temperature


A hot barrel is versatile barrel. One time when I was shooting my 223wssm Mongrel it got so hot that the bullet failed to seal the pressure expanding within the confined chamber. I pointed the muzzle down. The .224" bullet slid down the bore and exited teh muzzle. Not to give it rest, I simply fired 243wssm Mongrel II cartridges until they too fell out of the bore, so I stepped it up a notch and fired 25Wssm Mongrel III cartridges for a while and about the time I was about to fire a bigger bullet in my ever expanding bore, Ma ranged the dinner bell and well, if Mamma ain't happy, ain't nobody happy, so I went to dinner. Unfortunately for us I never got around to firing the larger projectile down the bore. But it was strange during my next outing. The ambient temperature was well below freazing. Sensing a trend of sorts, I took along 204" projectiles, and sure as hell, they fit well because the bore had shrunk in the colder ambient atmosphere of the range that day.
"Shazzaum! Shazzaum, Gomer said."

 

[Edds]

Re: Rifle barrel expansion with increasing temperature

The barrel will get bigger as it heats up.

 

[2Shots]

Re: Rifle barrel expansion with increasing temperature

Well that was a useless bit of drivel! Entertaining but no help. So can a bore get larger due to heat thus changing the friction and thus the speed?

 

[ezee]

Re: Rifle barrel expansion with increasing temperature

Yep, hope his day job isn't comedy!

 

[seanh]

Re: Rifle barrel expansion with increasing temperature

As a tubular piece of metal cools, the material shrinks (all the material moves towards each other)...ID moves out, OD moves in.

 

[ezee]

Re: Rifle barrel expansion with increasing temperature

Thanks seanh. I assume the converse is true. The ID moves in and the OD moves out as tubular steel heats. So net effect on mv as this occurs?

 

[Dylan in AZ]

Re: Rifle barrel expansion with increasing temperature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ezee</div><div class="ubbcode-body">Thanks seanh. I assume the converse is true. The ID moves in and the OD moves out as tubular steel heats. So net effect on mv as this occurs?</div></div>
I can't imagine it would be any measurable amount. Everything is made out of molecules and sure, when they are heated they bounce around much quicker causing the material in question to expand. Same thing happens the other way around; when said material cools down, the molecules slow down in movement and the material in question does shrink. But, this is all happening at the atomic level which is so incredibly small that such a slight change in heat will absolutely not have the effect that you are describing. If such a change was possible, then there would be MUCH more documentation and study on the topic both here and in other shooting forums, but the reality is that isn't how it works.

So to sum up; YES, the metal of your barrel does shrink/expand with increasing or dropping temperatures however the change is so microscopic that you will never notice it having any measurable effect on the velocity of your loads.

I'm not an engineer, a chemist, physicist, metallurgist, or anything of that sort but I do remember my high school physics class going over this topic. That being said, it was a while ago so there's always a chance that I'm wrong. So take it FWIW.

Hope this helps,
-Dylan

 

[Edds]

Re: Rifle barrel expansion with increasing temperature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ezee</div><div class="ubbcode-body">Thanks seanh. I assume the converse is true. The ID moves in and the OD moves out as tubular steel heats. So net effect on mv as this occurs? </div></div>

This is not correct. When it is heated the OD and the ID will get bigger.

 

[edkern]

Re: Rifle barrel expansion with increasing temperature

Thermal expansion will increase the outer diameter AND the inner diameter. So when a barrel cools the bore gets smaller, when it heats up it gets larger. Think of a square divided into nine pieces (3x3). Remove the center square and heat up the other squares, each of the eight remaining squares will expand, therefore the gap in the center will also expand. The same is true of the bore.

 

[wchartz]

Re: Rifle barrel expansion with increasing temperature

Ed Kern is correct. I own a fabrication and machine shop. We frequently heat rings, (i.e. ring gears on flywheel) so that they can be easily slipped over the solid shaft or flywheel. When they cool down there is an interference fit between the two parts. They will not slip when assembled this way.

 

[Uncut_Teeth]

Re: Rifle barrel expansion with increasing temperature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ezee</div><div class="ubbcode-body">Thanks seanh. I assume the converse is true. The ID moves in and the OD moves out as tubular steel heats. So net effect on mv as this occurs? </div></div>

No, it's the other way around. That's how interference fits work, in situations where a pin with an OD of, say, 1.002" fits into a hole with a diameter of 1.000". Heat up the hole, or dramatically cool down the pin.

 

[2Shots]

Re: Rifle barrel expansion with increasing temperature

Where this came up was when were doing chrono work yesterday at the range and the speeds were way off. For example my typical 115dtac load is 42.2 of H4350 and in the summer will run 3060 or so. I couldn't hit 2900 yesterday. Now the temperature swing is pretty severe, 90 deg compared to 40 but were talking almost 200 fps with a temp stable powder, come on is this normal? We are getting ready to shoot a match in a few weeks and want to check our dope. Our local range only has 385 yards and maybe 500 if you sneak on the normally closed range but we are pretty sure we will need good data out to a grand, last year it was out to 900 yds at this match. So any help here?

 

[sirhrmechanic]

Re: Rifle barrel expansion with increasing temperature

Barrel expansion is going to be a lot less than the effect of temperature on powder, primers, etc.

For example, rounds that are left in the sun (vs. kept in the shade under a cloth or in a pouch) are going to perform markedly differently. Your POI will absolutely shift.

This is the value of a databook and of writing down your shifts based on temperature, humidity, etc.

Up here we shoot in winter and have quals at times in -20F weather... and in summer it can easily be in the low 90's. That's a 110 degree temperature swing, not to mention the changes in humidity, etc.

I would not chalk it up to barrel expansion so much as the powder performing differently in different temps.

Also, remember that a barrel expands slightly under pressure. If you were taking high-speed video at an almost microscopic level, the barrel will swell. I've heard it described as watching an ostrich swallowing a tennisball as the bullet goes down the barrel, though not sure if that's exactly correct.

This pressure is going to have a much greater effect than the heat (unless you are talking about an MG barrel that's getting extremely hot.)

Cheers,

Sirhr