Actually you are wrong. There are many people who survive a gunshot wound to the head. There are people who stuck a gun in their mouth and pulled the trigger, but missed the spinal column--they end up with a hole in their neck, but not dead. There are people who are shot in the side of the head, but the bullet just cracks the skull and deflects. They end up with a brain injury, but not dead.
Bullets are spectacularly ineffective. They are good at killing people, sure, but in most cases the victim dies of blood loss, not an instant, painless death.
A well oiled guillotine is likely the quickest and most humane way, but it is a gruesome sight to see the head rolling into a bucket. On the other hand you can stick the bloody head up on a spike in the town square afterwards--hey, if you're going to engage in savage behavior why hide it?
First all I said was "bullets from a gun". I did not mention the head.
Second I said bullets not bullet.
If the shooter used a .45 or a 9mil double tap to the heart the job would get done to satisfation.

