Do I need to re-zero my rifle when switching from my TTC calibrated for 1000ft. above sea level to the one calibrated for 7000 ft.?
You should always re-zero a rifle when time and conditions permit. However, if you follow the recommended advice, you may only have to re-zero your rifle one more time.
For example, your rifle is currently zeroed for your home range (1000ft. above sea level @ 70° F). Make sure that you are dialed in at 100 yds and remove your low altitude calibrated TTC. Mount the one calibrated for high altitude setting it at 100yds. Shoot your rifle at a target placed at 100yds when you are at or close to the 7000 ft. altitude.
Example: You determine that the center of your 3-5 shot group is now 2 in. higher at 100yds. Rotate the knob two clicks (@1/4 in. per click) down (decrease yardage). Loosen the setscrews, reset your TTC to 100yds, and then tighten down the setscrews. Now you are zeroed for the hunt with the proper calibrated TTC. TTC knob swapping is always done dialed in at 100yds regardless of your zero range.
At the end of the hunt or when you get home you simply remount your low altitude knob setting it at 100 yds, add two clicks (increase yardage), loosen set screws and reset knob to 100yds.
By recording the required adjustment in a logbook or taping the data on the stock of the rifle you will never have to re-zero your rifle again. You will be able to continuously mount and remount different knobs.
Your logbook entry might read: 100yd zero @ 7000 ft = 2 in. high.
You might not even have to re-zero the rifle at 100yds, but simply switch knobs. If you are shooting a high velocity magnum cartridge, the altitude and/or temperature change will probably not have a significant enough effect on the trajectory of your bullet due to the short flight time out to the first 100yds.
If the changing field conditions did effect your 100yd zero with the center of your 3-5 shot group ﾽ in. higher, this change would place your shot only 2.5 inches higher at 500 yds. The kill zone area runs 16 to 24 inches depending on the size of the game animal. You may consider keeping it simple and just swapping knobs.
If your scope has 1 moa adjustment (1.047 inches of bullet impact change per click at 100yds), your click adjustment value might be greater than the actual bullet impact change at 100yds.
The size of your target and the distance you intend to shoot will determine how precise you need your 100 yd zero to be. The smaller the target and the farther the shot the more precise you have to be.
If the gunpowder used in your cartridge is extremely temperature sensitive you may loose enough muzzle velocity at extreme cold temperatures to substantially effect bullet impact at 100yds.
Because of a broad range of variables: the amount of increase in altitude, the drop in temperature, and your gunpowder’s sensitivity to temperature change which alters your cartridge’s muzzle velocity, it is best to re-zero your rifle the first time you are swapping knobs and record the results so it will be the last time!