A gun would be the most effective method that a normal person would have access to.
holocaust killed himself by shooting himself with a gun
I prefer the P90 sub-machine gun because it is accurate and has 50 rounds in one magazine. P90 Rocks!
Komers machine gun
The German machine guns that covered the beaches, particularly at Omaha.
One of the largest was the WWI "Paris Gun" developed by the Germans. It fired a 210lb shell about 81 miles. The gun weighed 256 tons. It was not very accurate but an effective psychological weapon.
A shot gun is more effective
Of course (while not entirely new) the proliferation of the machine gun forced the type of warfare that ensued.
Not Really
They took a gun to their heads to avoid listening to you. Wow...
The purpose of the military is to use force to defend a country. The most effective means of applying force is a gun.
Most western nations did not torture prisoners in the Holocaust.