Savvy
Those velocity averages were Chronographed at temperatures of between 65 and 70 degrees.
The rule of thumb with 175 gn 308's on Temperature is at "1000" yards for each change of one degree there is a corresponding change of 1 inch in rise or drop. So if the gun was zeroed at say 60 degree's and the temperature is now 70 degree's at the same altitude you can expect the bullet to hit 10 inches high. Same is true if that second shot is made at 50 degree's you would see a drop of 10 inches off your center. So when shooting keep your ammunition covered so that any direct sunlight doesn't heat up the shells. Warmer the ammunition higher the pressure created by the ignition of the gun powder.
Same is true of altitude. For every 100 ft change you can expect a corresponding change of 1 inch at 1000 yards. So if zeroed at sea level and your shooting in Denver (5K ft) at the same temperature you can expect the bullet to hit 50 inches high. That is due to the air density change. Humidity has little effect on bullet performance. It's there it just is insignificant to most shooters.
The spinning of the bullet or Magnus effect (right hand twist) also will cause the bullet to drift up to 24 inches to the right at 1000 yards.
The rotation of the earth also has and effect on the bullets path. This is called the Coriolis effect. It has no effect if shooting East to West or West to East. But if you shoot North to South or South to North it will effect the path or windage L to R.
Keeping the bullet supersonic makes sure that the accuracy is retained by keeping the air from effecting the trajectory. If you notice on a match rifle bullet there is a tiny hole or hollow point in the tip of the bullet. There is a reason for that design. When fired the bullet creates a tiny air bubble in that indentation.
That air bubble is a buffer between the sound barrier or wave in front and the bullet itself.
Once that sound barrier is lost the bullet begins to be effected by the air it is passing through. The bullet begins to yaw or wobble as both the rotating and forward speed decrease.
The type of bullet you use matters at long or short distance. For example my 300 win mag is lucky to keep a 2 inch group at 100 yards with 190 gn match ammunition.
Yet I have shot 5 shot 2 1/4 inch - 2 1/2 groups at 700 yards with the same gun and ammunition. The reason is when the bullet leaves the muzzle it yaws or wobbles for a period of time before it settles down due to the effect of spinning.
If I shoot 155 gn factory hunting rounds it will group easy under 1/2 moa at 100-200 yards.
Let me add I have a separate Data book for each of my long range rifles.
Data Book
[ February 16, 2006, 04:27 PM: Message edited by: ALS ]