head and tailwinds can affect your point of impact up and down another thing to think about is when your bullets transitions from supersonic to subsonic your b.c. can vary a little
its called pit run there rocks that they use to build new logging roads with there not crushed as small as say your driveway gravel and i just pick up some 6-8 inch ones and pack them out into my favorite shooting range(old logging clearcut) and place them on the stumps at random distances then...
you said something about face shot and i had a freind who shot a deer in the face from 20 yds with a 30-30 and they thought it was dead. when they went back to get it the deer attacked them cause they thought it was dead all it did was knock it out.