Question About Cooling Rate in Gun Barrels

[Welding Rod]

Conventional thought in the gun forums is that a rifle barrel that is smaller in diameter will cool faster than an otherwise identical gun barrel that is larger in diameter, after the same number of shots fired.

Is this true?

I have trouble understanding how it could be since the larger diameter barrel would have more surface area in contact with the air.

 

[Curl]

Small barrel means there is lower heat capacity. After the same number of shots, the small barrel will be hotter. Hot things cool off faster than cold things (cooling rate is proportional to temperature difference).

 

[Welding Rod]

Thanks Curl

During cooling, once the temp becomes equal in each, would the larger diameter barrel have a cooling rate advantage from that point on? Or would it still cool slower since it was holding more heat in its greater mass?

 

[turbo]

Years back, I was trying to pick up Bushmaster as a client, and talked to their designer while we toured the plant. He liked fluted barrels. They retained stiffness like a larger barrel, but the fluting gave the barrels lots of surface area for better cooling.

 

[JeffKoch]

Depends what you mean by cool faster - reach the same temperature, or rate of change of temperature, or...? Does the wall thickness remain constant as the outer diameter increases, implying larger bullets and more powder? But generally outer surface area scales as R and inner volume (and therefore amount of heat input from powder) increases as R2, so you would expect a thinner barrel to cool more rapidly. Adding fins and whatnot to radiate more heat would make any barrel cool more rapidly, too.

 

[Welding Rod]

Hi Jeff

Once the outside temp of the smaller diameter barrel is the same as the larger diameter barrel, which will cool further, to ambient temperature faster?

The question is for otherwise identical barrels, just different outside diameters. Therefore same bore diameter, number and size of lands and grooves, cartridge chambering, powder level, etc. The wall thicknesses would therefore be different.

 

[mrspeedybob]

Hi Jeff

Once the outside temp of the smaller diameter barrel is the same as the larger diameter barrel, which will cool further, to ambient temperature faster?

The question is for otherwise identical barrels, just different outside diameters. Therefore same bore diameter, number and size of lands and grooves, cartridge chambering, powder level, etc. The wall thicknesses would therefore be different.

In this scenario the smaller barrel will cool faster because the ratio of the surface area to the volume of metal in is R/(R²-r²) where R is the outside radius and r is the inside radius. Since you stated that r would be the same in both cases as R increases the ratio decreases meaning the barrel takes longer to cool from the same temperature.

However, is the real question about rate of cooling or resistance to overheating? If it is about resistance to overheating then the thicker barrel is without doubt superior. If both guns shoot the same number of the same round in the same time then the same heat energy will be input into both barrels. The larger barrel will not be as hot because there is more mass to soak up the heat and more surface area to radiate it. In addition the thicker barrel will fail at a higher temperature then the thinner one because the added thickness of the metal will help the barrel maintain its rigidity longer as the metal softens.

So, if this is a machine gun and the question is "how many round can you fire in how short a time before the barrel melts?" then the larger barrel is better. If this is a hand gun and the question is "how soon after firing a 15 round magazine can I put the gun back in my pocket without burning my leg?" then the thinner barrel wins.

 

[Welding Rod]

Thanks Mrspeedybob. Makes sense.

I was most curious about the actual cooling rate. I shoot a lot and have seen fatter barrels shoot well during sustained fire while thinner ones would shift point of impact and/or open up the groups significantly. Of course no two barrels are manufactured to identical quality standards either.

I have also noticed fat barrels tend to shoot all loads reasonably well while thinner ones seem to be more particular. The fatter ones have also tended to put differing loads close to the same point of impact while thinner ones have experienced greater point of impact shift when switching loads.