I shoot from my deck to a berm 100 yards from my deck. Due to the contour of the ground and the height of my deck, I cannot get my chrony closer than 35 feet from the muzzle. Yes, I have to walk over shooting. My question is how much velocity is lost between 15 feet and 35 feet. I realize that this will vary from rifle to rifle and for other factors such as ballistic coefficient of the bullet. I am wondering if there is a formula that could be used if factors such as weight, bc and other data were plugged in. The reason I ask is that I shot my 416 Rigby and it was showing an average of 2480 for six shots using 370 grain North Forks.

Don't know the formula offhand but some ballistics programs will correct for that based on BC and recorded velocity. Otherwise if you get two velocity readings you have a starting point for figuring it out. My guess would be around +30 to 35 fps.

I don't know the Ballistic Co-Efficient for the 370 gr. NF, but I guessed .360, and am probably close, as it looks like a semi-spitzer profile. Your muzzle speed should be around 2510 fps. This should be close.

http://www.eskimo.com/~jbm/ballistics/traj/traj.html