txhunter58,

It is the factor of less air density and humidity that make up the factors in which make the bullet retain velocity which produces ft lbs of energy and the trajectory flatter. Theres also barometric pressure change factors. Which goes something like this 1" of change of barometric pressure equals somewhere around a 1000ft of elevation change. Also there's the hot and cold factors. But just for the sake of the rest of the readers of the column I won't bother describing those. You can find all that out through research.

I have several computer ballistics programs. Tons of reloading manuals. And many days at the range testing out stupid crap just because I read about it somewhere. lol Most of it you can through out the window. But not checking your zero when changing serious elevation could be a major factor on a longer shot.

I do chronograph at 300yds using a 1" steel plate cut out to shoot through that has a 6" hole in it. I also use 1/8 dowel rods inplace of the metal ones for the just in case opp's. I also shoot for groups upto 500yds every year no matter how many times I've loaded the same load or shot the same rifle I like to know exactly where the rifle shoots before taking off on my hunts.