A gun would be the most effective method that a normal person would have access to.
I prefer the P90 sub-machine gun because it is accurate and has 50 rounds in one magazine. P90 Rocks!
holocaust killed himself by shooting himself with a gun
Komers machine gun
The German machine guns that covered the beaches, particularly at Omaha.
A shot gun is more effective
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.
Of course (while not entirely new) the proliferation of the machine gun forced the type of warfare that ensued.
Not Really
The purpose of the military is to use force to defend a country. The most effective means of applying force is a gun.
The Machine gun was the most effective weapon in world war one. It killed more than 7million people out of 8 1/2million. Used to protect trenches. It was effective enough that a counter measure had to be developed to overcome it. The tank. Even in WW2, Gen Patton stated that the primary weapon of a tank was it's machine gun, and not it's cannon.Was and still is.
We need you to define "effective" and "6 gun". If you are asking the maximum effective range of a Colt Single Action Army revolver, sometimes called a six shooter, it is about 50 yards.