Re: How much does your rifle's point of impact change when you travel?
You need to reset a zero if your elevation changes by 1000 feet and your ambient temperature by 20 degrees Fahrenheit, especially at long range. Would not be a factor in short range brush hunting.
Heat mirage is a major factor around here on the prairie, but nothing to do with rifle, scope or ammo. I just aim a bit lower when I detect heat mirage.
Powder sensitivity to temperature is a consideration, as well as primer sensitivity to temp change. People rarely consider this when working up a load for the smallest groups at the rifle range. Word on the street was that Winchester primers had the least sensitivity to temp change, so I bought thousands of them way back and am still shooting off them, except for some recent CCI magnum primers. Military powders seem rather stable with temp changes, in general.
If powder and primer are changing with temp, you don't have an issue with rifle/scope point of impact changing. Only time I had an issue with my rifle changing point of impact was when I had a failing scope.
I learned the hard way that what shoots great at one temp might not do the same at a different temp. It's not a rifle issue but an ammo issue. I live in a continental climate with wide seasonal temp changes, not such a big deal if you are coastal.