Call of Duty 4 Challenge help (not cheats)

[deathchaoz]

How do I do the Think Fast challenges?

I've tried directly hitting people with the bombs after shooting them twice with a pistol and it doesn't kill them with a direct hit on them,, just bounces off then the explosion kills.

[Rmstrjim]

They gotta be more dead.
I got lucky and managed one by accident. /shrug

[deathchaoz]

Doesn't even kill them when they are flashing red, just bounces off them. :(

:EDIT:

Do it while they are in last stand, doh.