Why does muzzle velocity increase with barrel heat

[AKguy1985]

You'd hafta shoot atleast 120 rounds through an AR rapid fire to get a cook off. Thats $60 in ammo aint nobody can afford that.

 

[doctorjj]

As the barrel heats, it expands, making the bore smaller in diameter than a cool one, therefore increasing the pressure behind the bullet to push it out, as stated above.

Wrong! As a barrel heats up the bore expands.

 

[n2sooners]

From my experience with heating and cooling when it comes to installing bearings, I'm gonna go with the barrel expanding on the inside and any increase in velocity would be caused by reduced friction on the bullet.

That's my guess.

 

[Old Fart]

Friction causes heat.

 

[CHenry]

Wrong! As a barrel heats up the bore expands.

No, your wrong. When you have one of them dougnut shaped pool floaties this summer. Put normal air in it like you would to use it. Measure the "bore" diameter. Now blow it up more and measure again. It expands inward making the diameter smaller. The expansion occurs evenly to both inside and outside of the steel. Don't let the fact that it's a closed circle confuse.

 

[doctorjj]

No, your wrong. When you have one of them dougnut shaped pool floaties this summer. Put normal air in it like you would to use it. Measure the "bore" diameter. Now blow it up more and measure again. It expands inward making the diameter smaller. The expansion occurs evenly to both inside and outside of the steel. Don't let the fact that it's a closed circle confuse.

Haha!! Take a physics class and then come back.

 

[Okie4570]

Wrong! As a barrel heats up the bore expands.

So you're saying that ID and OD get larger and that all expansion is outward? What's the physics behind no ID expansion? When driving in keys or knocking out old ones on farm machinery, we do heat the hole to drive them in or out easier, so maybe I am wrong, it would not be the first time. This would mean that a solid cylindrical object, when heated, can only expand outward, making its diameter larger. That makes sense. Where as a tube, when heated should want to do the same on the outside, but you're saying it doesn't expand inward at all, that it contracts.........the inside movement follows the outside?

 

[oneof79]

There hve been guys shoot machine guns until the barrel is red hot. By then a 30 cal barrel would be swollen down to 22 cal or some such, it doesn't happen. Maybe the increase in velocity is from the fouling incurred in the first few shots, and even this has limits.

 

[n2sooners]

Here's a video showing the basic concept. When heating a cylinder (in this case the inside race of a bearing) the inside diameter expands. And in this case, the expansion is enough to make a bearing that would otherwise have to be pressed onto the shaft just drop on instead.

[video=youtube_share;GJM8dnd_ptg]http://youtu.be/GJM8dnd_ptg[/video]

 

[doctorjj]

So you're saying that ID and OD get larger and that all expansion is outward? What's the physics behind no ID expansion? When driving in keys or knocking out old ones on farm machinery, we do heat the hole to drive them in or out easier, so maybe I am wrong, it would not be the first time. This would mean that a solid cylindrical object, when heated, can only expand outward, making its diameter larger. That makes sense. Where as a tube, when heated should want to do the same on the outside, but you're saying it doesn't expand inward at all, that it contracts.........the inside movement follows the outside?

For the ID to decrease in size, then molecules lining the innermost aspect of the barrel would have to collapse against themselves. When heated, the exact opposite is what occurs. Those molecules expand, thus increasing the diameter of the bore.