Regular heavies or heavy heavies? Seems like sometimes I can drop a helmeted enemy with one shot in M&E while the guy he's standing next to just gets his helmet pinged off. Not sure what exactly is at work here - whether its weapon power/distance, angle, or both.
Both. Sam will grab them at the beginning of the animation. perform the any other executes you have marked, and then snap there neck.
I always try to have meter to mark heavy heavies when I am sneaking up behind them so that even if they hear you and turn around you can get an instant melee kill before they alert anyone else by burning your execute meter,