what calibers are best? i have heard .25 and .30? is shot placement different than with a powder burner. i see youtube video's of guys shooting squirrels with 25-30 and they head shoot them?
If you want to do it right (Legal) and keep your hunting privileges, you will need to check your States' Regulations. Most states, like mine (Texas) require a minimum caliber (0.30 cal, min.), bullet weigth (150 gr, min.) , velocity (800 fps min.) and/or hence a min. power rating (250 ft-lbs), which pretty much nixes whitetail for me, but that's okay... because I've shot plenty in my lifetime. Not as good tasting as other options....
For shooting deer with an air rifle, I go after the non-indigenous species that are not affected by the Texas Regs; Axis deer, Fallow deer and Sika are plentiful in my neck of the woods. A 0.30 cal. Daystate and proper shot placement (immediately
below and front of the ear) will drop'em like a limp noodle everytim; Dead on the spot.
Going for the heart or lungs isn't an option for me, and I personnally would not recommend this for air rifles. I don't like wounding or hurting the animal. Instant kill is the only way. WIth that said, shot placement is critical when hunting big game with an air rifle... (centerfire and rimfire too, but these are obviously more forgiving). Don't screw around, zero your gun and get a range finder. It's a must for serious air rifle hunting.