WOW that's disturbing news! Usually when a top predator like a fully grown Mtn. Lion stalks you, you don't know it until it's ON you like a fuzz covered buzz saw! Sure glad you saw it, I bet those memories will be around awhile!

From 1985-2006 I hunted around Mtn. View, MO but never saw any cats around there. (Other than THE largest tom bobcat I've EVER seen in my life!) But I understand now there are several confirmed pics and encounters with cougars in those counties now. I can see how sitting quietly under a tree, motionless in camo while mimicking a turkey would be like a baited hook for a big cat!

As for your question, I doubt a single 20yd shot would kill a big Tom but I bet it'd change his mind about continuing his attack. If he was facing you then likely he'd at the least be blinded. Inside of 10yds yeah I bet a head shot WOULD likely kill him, if not instantly then damage such as blindness and maybe a broken jaw would likely take that much out of his arsenal. Add in your 2 remaining shots and I seriously doubt that ol Tom would still be hellbent on rearranging your anatomy! If you EVER have another encounter and DO have to shoot then DON'T shoot for the body, make sure and go for the head. (Assuming he's stalking/stationary, if he's on a full run and about to take a hunk out of you then obviously you put the lead in him wherever it'll land!)