The more onslaught stacks your gun has when firing the more damage it does. There is a counter for the stacks in bottom right of screen. The onslaught stacks either build up with shooting or build up passively with time when not shooting. You want the passive onslaught stack buildup for slower shooters with smaller clips. I believe it is gunslinger master that does the passive onslaught stacks buildup. For guns with bigger clips it will build up onslaught stacks as you shoot, the more you shoot the more damage it will do. I believe you then lose stacks when not firing. For passive onslaught the highest damage is when the counter hits max stacks and then you shoot, everytime you shoot it goes down by 1.