Thread: Effects of cartridge temp on velocity

Originally Posted by sneeze

Once its established you dont have to enter it again, it just works of the atmo.
Excluding altitude,atmospheric change seems to cancel itself out in many situations as higher pressure usually comes with warmer temps leveling out air density change

Frosty mornings being the exception to that.

Originally Posted by Shearer

Frosty mornings being the exception to that.

Yes thats why I said "many situations" not All.

Originally Posted by stug

How about this, how does the air pressure effect the muzzle velocity? Does it have any effect. My first thought is that a lower air pressure (higher air temp) would give a higher muzzle velocity because there is less resistance in front of the projectile in the barrel. Or is it such a small factor the friction between projectile and barrel negate it?

I read just yesterday on a US forum a guy tested this. His smith built his rifle and testfired it at sea level, the rifle and remainder of the box of ammo was then shipped it to him, he lives at 4000 feet and when he tested the left over ammo he got the same velocity as the smith.

Originally Posted by wsm junkie

I read just yesterday on a US forum a guy tested this. His smith built his rifle and testfired it at sea level, the rifle and remainder of the box of ammo was then shipped it to him, he lives at 4000 feet and when he tested the left over ammo he got the same velocity as the smith.

Altitude simply effects air density. So does temperature. Temperature normally decreases with altitude (the amount varies depending on humidity) so they were probably shooting in the same "thickness" of air.

Originally Posted by Shearer

Altitude simply effects air density. So does temperature. Temperature normally decreases with altitude (the amount varies depending on humidity) so they were probably shooting in the same "thickness" of air.

Dont forget air pressure lapses with altitude as well at a given rate per 100ft.

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Originally Posted by R93

Dont forget air pressure lapses with altitude as well at a given rate per 100ft.

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Yeah. That's what I am thinking. As the altitude increases the pressure decreases (because of reducing air density) but the temperature also decreases (increasing air density) but not necessarily at the same rate.
Please let a scientist clarify this before I talk myself into a hole.
In the example given, air will cool between 1.5 and 3 degrees /1000' of altitude (depending on it's saturation level), so a rise of 4000" altitude will equate to a temperature drop of between 6 and 12 degrees Celsius.

Originally Posted by Shearer

Yeah. That's what I am thinking. As the altitude increases the pressure decreases (because of reducing air density) but the temperature also decreases (increasing air density) but not necessarily at the same rate.
Please let a scientist clarify this before I talk myself into a hole.
In the example given, air will cool between 1.5 and 3 degrees /1000' of altitude, so a rise of 4000" altitude will equate to a temperature drop of between 6 and 12 degrees Celsius.

Ha Ha you did alright. I passed my met exam that was full of all that stuff a while ago. I can't remember we the formula as I just went off QNH or dialed my know altitude on the altimeter

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Being a sample size of one, shot on different days by different shooters with unknown chronys or method of measurement Id say its a conclusion of very limited value.

Originally Posted by sneeze

Being a sample size of one, shot on different days by different shooters with unknown chronys or method of measurement Id say its a conclusion of very limited value.

Yeah. Not a lot of detail.

Originally Posted by Shearer

Yeah. That's what I am thinking. As the altitude increases the pressure decreases (because of reducing air density) but the temperature also decreases (increasing air density) but not necessarily at the same rate.
Please let a scientist clarify this before I talk myself into a hole.
In the example given, air will cool between 1.5 and 3 degrees /1000' of altitude, so a rise of 4000" altitude will equate to a temperature drop of between 6 and 12 degrees Celsius.

The main thing, I think, is that the lower temperature reduces MV. This largely offsets the lower pressure/air density so the trajectory is much the same.