I am not even too fond of the whole bullet to the head option. We have these devices that are like guns that launch a large spike into mammalian livestock's heads when it is time to slaughter them. Its quick, efficient, as painless and humane as reasonable and it is reusable.
Bullets fired from guns are really too unpredictable, the most efficient would be something like a guillotine, but instead of severing the head from the body, it instead completely crushes the head. No chance of survival, and death is instantaneous. Bullets can do all sorts of things like passing straight through and doing minimal lethal damage, to bouncing off the skull and not directly damaging the brain. It does not happen often, but it can.
Of course, personally I believe a lot of problems in the world can be solved with the wholesale slaughter of significant portions of humanity. We really do need to cull the herd a bit. Unfortunately I would probably be one of those culled, so I cant really support it. In life there are many ugly truths, that go against our own self interest.