The Darkness At The Edge Of The Galaxy
Every weapon has a critical hit ratio. Basically this is the percent chance that a gun has of achieving a headshot, this increases damage done to the target.
Upon making an Attack Roll you first determine whether or not the attack was a success and the shots hit. Then you determine which shots, if any, were critical hits by comparing them to the critical hit chance, starting from 1 and going up.
Each weapon has a critical damage modifier which dictates how much extra damage is dealt by the critical hit.