Luckily, MOA is always constant and so is IPHY. So no matter what elevation you're at, if you move your scope 4 clicks, it will move the reticle 1 inch at 100 yards (if the scope is a .25" IPHY scope) if the scope is working as it should. In other words, the internals of the scope are not affected by elevation change.
However, your external ballistics change significantly with altitude change. Bullets fly through the air better at higher elevations. So the key is to have your rifle properly bedded and the barrel free floated so that the point of impact remains the same and functions similiarly to the scope in that it does the same thing every time. This ensures that there won't be impact change due to equipment.
At 100 yards, I have never seen a significant change in trajectory at upper altitudes. It WILL BE THERE but most guns can not shoot accurately enough to detect it. It may be less than a bullet's radius. But when you add time to the trajectory (or shoot farther distances in other words) the differences become more apparent. When shooting at excesses of 11,000 feet, I may see minutes of difference in trajectory in shots of 1/2 mile and farther depending on the particular bullet. But POI at 100 yards usually stays within several thousandths of where it was at lower elevations.
So, what I do is make my gun hit at say 3" high at 100 yards at my home elevation. Then run a ballistic chart and enter this information. It will then spit out at "zero" yardage. Let's say it ends up being a zero of 310 yards. Then if I decide to go up to 10,000 feet, I simply run the chart with my new elevation but reconfigure my zero and adapt the chart for the new zero which may be now 320 yards while my 3" high poi at 100 yards stays the same. If you don't reconfigure your zero and keep your poi the same at 100, the program will automatically readjust the entire parabola before the zero AND after the zero. Or in other words, it will keep the 310 yard zero but it will change the POI at 100 yards to something like 2.8" and your gun will not shoot .2" LOWER at 100 yards at higher elevation obviously. If anything, it will shoot .2" higher at higher altitudes.
So the computer will give you false information if you let it.
Clear as mud?
If it's not far, it's boring.
Last edited by goodgrouper; 12-19-2007 at 01:13 AM.