In most attempts, you are trying to do something that is opposed by something or someone else. But in some cases, you should be able to willingly fail or willingly succeed at something. Here's an example;
...a man can drop a grenade at his feet, just stand there until it goes off...
Dropping a grenade at your feet, I'd treat as automatic success for a normally thrown weapon-you can choose where the grenade lands and thus can get the best result possible. I'd treat it as a roll of +4.
Standing there until it goes off, I'd treat as a willing failure on your dodge, getting the worst result possible. That's a dodge of 0 rolling a -4 mechanically.
End result is, assuming you don't know how to place the grenade for extra damage, 16 shifts of stress.
Now, another example;
shooting a bound man in the head with a gun. Following similar assumptions of perfect shot/can't dodge, that's 10 shifts of stress.
My problem here is consequences.
In the first case, if you are lucky or reflexively try to cover yourself, you could still potentially take consequences and not be killed by the blow. In the second case though, how can one justify taking consequences for not being killed?