teddy:
I presume you are aware of the trajectory variation from altitude and temperature. A rifle sighted in at the altitude of Indiana is not going to be hitting the target at the same place at 9000 feet.
It isn't a big difference but something to be aware of.
It really doesn't amount too much in most cases, especially since most shots are still taken under 300 yards even when the hunter is prepared for longer. (Excluding situations in which the hunter is purposely TRYING to take a long shot.) Take one of my loads, a .308 Win pushing a 168g TTSX at 2650fps and zeroed for 250 yards, then look at the difference between 500ft and 9000ft altitude:
.308 Win, 168g TTSX @ 2650fps, 250 yard zero
500 Feet
- 4.6" @ 300
-19.5" @ 400
-43.2" @ 500
9000 Feet
- 4.2" @ 300
-17.9" @ 400
-39.0" @ 500
So, in the worst case, we're looking at a 4.2" difference at 500 yards when changing from 500 feet to 9000 feet in altitude. Crank the velocity up to 2900fps, the velocity of my .30-06 loads, and the difference at 500 yards is 3.7". Crank it up to 3090fps, the velocity of one of my .300 Win Mag loads, and the difference in drop at 500 yards is right at 3.2"
The key here is that the difference in drop is easily calculated and accounted for.
Personally, I don't zero for fixed ranges, I zero for Maximum Point Blank Range (MPBR) for a 6" target (maximum 3" rise and/or drop). Using the same loads:
.308 Win, 168g TTSX @ 2650fps, MPBR zero for 6” diameter target
500 Feet
265yds = MPBR Zero
- 6.7" @ 300
-22.4" @ 400
-46.8" @ 500
9000 Feet
273yds = MPBR Zero
- 5.7" @ 300
-19.8" @ 400
-41.4" @ 500
Here the worst case difference is 5.4” at 500 yards for the .308 Win. With the 2900fps .30-06 load it is 4.3” and with the 2090fps .300 WM load it is 3.6”. Again, easily calculated and accounted for. In fact, if one was to estimate the difference in drop as 1” @ 300, 2” @ 300 and 4” at 500, , one would never be more than .5” off at 300 yards and .6” off at 400 yards. At 500 yards the .308 Win would shoot 1.4" lower than estimated but the .30-06 and .300 Win Mag would both be within .4” of the estimate.
Given the above, it isn’t even necessary to re-zero – just calculate new drop tables. One should, of course, always check the zero after making a lengthy trip. Without re-zeroing, one would just check it against the new drop tables -- preferably at 300 yards or more.
In my case, I shoot all year at a range where the long-range rifle range sits at 6237 feet with MPBR zeros. When shooting at 9000 feet, the calculated differences for 500 yards are :
1.5” for .308 Win
1.4” for .30-06
1.0” for .300 Win Mag
At 500 yards, those are differences I can ignore.