View source for Bayonets & Melee Bash

Most weapons in game are capable of performing a melee attack with a few exceptions. Melee attacks are the last resort for a soldier with no ammo or a way to kill an enemy without making any noise. 

Pressing the melee button(Middle Mouse by default) will perform a quick melee bash with your weapon. Every weapon has same damage when bashing, only the range is different. A melee bash will not typically kill an opponent outright unless they have already taken damage or you hit the head. Alternatively the player can hold down the melee key to charge up an attack, resulting in a one-hit kill.

Some weapons have the option to mount a bayonet. Bayonets increase the range of melee attacks and are instant kills as long as you do not hit the limbs.