ORIGINAL: eldeguello
We got into a discussion about sighting in scopes and he made a comment that in warmer weather, the heat rising from the ground will cause the bullet to rise (maybe I should of asked him if a bullet sinks faster because of the cold air, just to get a laugh).
Well, a bullet DOES actually drop faster in cold air vs warm air. The reason is that cold air is denser, and therefore provides more resistance to the bullet's passage, thus slowing it down faster. (Think of it as lowering the bullet's B.C. a little.) This means that the trajectory curve will be a little steeper when fired in cold air vs warm air. This is one of the reasons artillery firing calculations include both air temperature and local atmospheric pressure in the variables.
Good point, that plus what DM brought up:
Another thing, that i've SEEN happen is, ammo fired in HOT weather, the powder makes more pressure (pressure = fps) and so shoots slightly flatter. Shoot the same ammo in very cold weather and the V is slower!!
This means if you sighted-in in July in 90 deg weather you'd better check your point of impact at the range closer to hunting season when you might be shooting in 10 degree weather!
Now I'm curious, has anyone got any chronograph logs? What I'm wondering is if the chamber temperature (heating up as the gun gets fired over and over) has a noticable effect on velocity and there by affecting the point of impact. This may also be a factor for sighting in for hunting season because your deer is not going to wait there patiently while you fire off a few rounds to warm up the gun to the same temp it was when you put that final adjustment on your scope...LOL!