'It soon became clear to soldiers that the rifle was not the best weapon for combat inside the trenches. Where it could it kill at great distances an enemy upon the open battlefield, during the day, in the dark of the night, at close proximity inside the confined earthen entrenchments, it was almost useless.
The weapon that is least needed is a rifle. A club or a sandbag or an Indian battle axe or spiked club is better. A good slugger without any weapon at all may take an adversary's loaded rifle away from him and knock him down and then kick him to death.

The preferred firearm of the trench fighters were the shotgun and revolver. But these were less desired than even more primitive ones, since their use would alert the enemy. In this most modern of war, soldiers quickly relearned the lesson of their medieval predecessors, arming themselves with an odd assortment of mélée weapon with which to butcher their opponents.
Raiders sallied forth from their underground dugouts armed with various bayonets, swords, hatchets, clubs, coshes, knobkerries, truncheons, hammers, daggers, pick-axes, push-knives, staves, and steel bars. The edges of entrenching shovels were ground to razor sharpness. Trench raiding clubs were both homemade and mass produced, some of which were lead filled, had steel studs or spikes hammered into them, or had their heads wrapped in barbed wire. Men carried brass knuckles (or "knuckle-dusters") and a wide array of knifes with them. Sometimes these were combined: the US army issued trench-knives fitted with metal knuckle guards and "skull crusher" or "walnut-opener" pommels to their infantry.
Inside the trenches centuries of advancement in warfare was being discarded in favor of the older lessons of mélée combat.'
John S Nash on WW1.