[Willie Green]

but there's far less chance of something going wrong, and it's quick;

I'm not so sure about that...
not when they pin the target over the heart...
Yeah, that'll work, but no doubt it'd be quicker and more certain with a head shot.

[wimpycat]

They usually have several riflemen firing at once.

[El Gato]

quicker and more certain with a head shot.

Maybe, but bullets have been known to glance off the skull, and you might lose part of the brain, but not necessarily the part that keeps you alive. Take out the heart and the killer will die, maybe not instantly, but loss of conciousness will be rapid.