I think Jeremy panicked with all the bullets he had fired thus far along with the bicycle journey and the influence of the drugs and misplaced the first shot to Sheila,whereupon she was stunned and involuntarily moved her finger to the wound,hence the blood on the hand. I would have thought it would have been the easiest thing in the world to position the second shot so that it killed Sheila instantaneously.
The rifle would have had to have been on her person to have receieved a shot like that last one ( close proximity ) to have entered the brain.
Blimey,Steve,,Jeremy would have had to have been a contortionist to deliver such a shot.