Critical Hits or more commonly know as knockback, is the instance when a player is knocked down, causing the player to become unable to move temporarily.
Gurt states that chance measurements are determined in fractions. To simplify it's to be measured in precentages.
The only weapon to be have an absolute critical hit is the Bazooka which utilizes Explosions. Explosions always have 100% Critical Hit chance to anyone close enough to the radius.
Critical rates are higher depending on damage values. Meaning critical chance increases if damage increases. Example: The Pistol deals around 5 - 10 HP damage and has a 5% critical chance.
The one exception would be the automatic rate of fire of some weapons would reduce the effectiveness of a successful critical hit.