Given that a shotgun is a long gun, it would be difficult to place it to the head (awkward position and more could go wrong doing it that way), therefore, putting the barrel into the mouth is the most logical way to go about it. Also, with the barrel in the mouth, it is better to aim further back and at an low angle (15-30 degrees) towards the brainstem. 00 buckshot is the right way to go and just remember that when the gun goes off, you won't hear it or sense recoil as you would be unconscious (and most likely dead) after the pellets leave the barrel.