Why does muzzle velocity increase with barrel heat

[jmike314]

My $.02:

If you take the shape into account, it would be difficult for the ID to decrease as the material would have to expand into itself...so to speak. Expansion would come from the heated molecules' increased movement. There would be less resistance outward and therefore the direction of expansion.

As far as velocity increasing...does the heat increase in the barrel due to friction affect the powder in the cartridge? Would the increased heat cause the powder to burn faster and propel the bullet at faster velocity?

 

[ASP785]

I'll add my experience. In college, I worked on an assembly line at Mercruiser installing wrist pins into piston rods. Being an interference fit, you had to heat the rod up using an induction type heater and then slide the wrist pin into the piston rod. Not a fun job for 10 hours a day, but the ID of that rod sure expanded.

 

[CHenry]

My $.02:

If you take the shape into account, it would be difficult for the ID to decrease as the material would have to expand into itself...so to speak.

This makes sense. So based on shape, your saying if the barrel were split down the middle and hammered flat, it would
then expand outward in all directions but not when in a circular shape?

 

[JamesP82]

I had to use a heat gun to increase the ID on an upper receiver to fit the barrel on my last build. Metal expands outward if cylindrical.

 

[doctorjj]

This makes sense. So based on shape, your saying if the barrel were split down the middle and hammered flat, it would
then expand outward in all directions but not when in a circular shape?

Correct.