My checklist too would include:
Scope ring screws
Action screws torqued evenly and tight ...
Barrel heat would be waaaaay down the list for the kind of off target strikes you describe.
Sand bags are, IMO, perhaps the worst type of rest for load testing. But if you don't have a good rest (something like the Caldwell Rock) then sand bags will have to do.
How are you supporting the butt of the rifle?
Where is your left hand when you fire the rifle?
Flyers are often associated with rear bag placement/pressure. When a hand is used in place of the rear bag the problem can occur there too.
PETA influences don't make you miss; they blow up your rifle and Ex-Wives just bend the barrel so the muzzle points back in your directtion.
I have a great woman, fantastic kids, a warm place to sleep and an accurate rifle. Life is good ..............
Hunter Safety Instructor - Rifle/Pistol Marksmanship Instructor - NRA Life Member
American rifleman's triad - God, guts and guns. It built America and it'll preserve America. Abandon one and you lose them all.