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Lower velocity with warm barrel


tedahlenius85

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I've noticed quite a loss in velocity when I chrono'd my ammo when I get to the range and shoot a cold barrel vs after a couple of drills when my barrel is heated up. I guess its expanding and ammo is loosing pressure right? But what is "normal" loss with a warm barrel? My 124gr fmj runs at about 1070fps with a cold barrel, and drops about 50-60fps when I chrono after doing some shooting and barrel is quite hot. In my opinion thats more than to expect. I didn't even notice any drop running my other gun, but my current Stock 2 seems to be heat sensitive when it comes to pushing out those rounds.

 

Would appreciate your experience regarding this.

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3 hours ago, tedahlenius85 said:

I've noticed quite a loss in velocity after my barrel is heated up.

 

I haven't tried it, but I would expect as the barrel heats, the barrel would expand,

tightening the bore slightly and elevating velocity .....  ???

 

I'll have to try it one day ...

 

Are you considering shot to shot variation in velocity?

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On 4/26/2017 at 1:22 PM, Hi-Power Jack said:

I would expect as the barrel heats, the barrel would expand,

tightening the bore slightly and elevating velocity .....  ???

The barrel bore would expand causing it to loosen up.

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1 hour ago, TDA said:

The barrel bore would expand causing it to loosen up.

 

You might be right, but I'd think the metal would expand, causing

the hole (bore) to shrink because the metal expanded ....   ???

 

I'm not sure how a bore (hole) would expand because the metal

got warmer.

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It could be the powder, is it one of the ones that is reverse temp sensitive? I would get similar results when I loaded longshot in super for open. As the gun got hot, or just the weather got hot the pressure would increase and cause the velocity to drop.


Same with VV N320 & N330. And those are not supposed to be heat sensitive like that as far as I know.
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3 hours ago, Hi-Power Jack said:

 

You might be right, but I'd think the metal would expand, causing

the hole (bore) to shrink because the metal expanded ....   ???

 

I'm not sure how a bore (hole) would expand because the metal

got warmer.

A 9mm barrel would need about a 450 degree increase in temperature from ambient for the bore to expand .001".

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On 04/28/2017 at 6:11 PM, TDA said:

A 9mm barrel would need about a 450 degree increase in temperature from ambient for the bore to expand .001".

If i did the math right... the radius increases 0.001" in 450C but the diameter increases 0.001" at 1/2 that delta T.

 

Or did  mess up? 

 

Immaterial anyway as the projectile would probably heat up as well and minimize the mismatch. Not that we run enough to heat it to either extreme.

 

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Chemically speaking, the energy yield from the powder reaction is going to depend on the difference between starting and ending temperatures. The higher the starting temperature, the less temperature difference, the less energy you get from the reaction. It would be a very small effect, but I'm guessing 50 fps is a small percentage of your observed velocity. If this is the cause, there's nothing you can do about it. outside of waiting until the barrel cools down.

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