Not all of Zane’s kill skills stack. For example, Playing Dirty doesn’t stack.
I don’t think that’s what it says. Regardless of that, what it means is that whenever PD activates then the next 5 shots fired will have the PD effect. For single pellet weapons the PD effect is that 2 pellets are fired (i.e., PD doubles the damage of single pellet weapons). For multi pellet weapons (e.g., shotguns) it means an additional pellet is fired. For example, if a gun’s card says 356x3 the PD effect turns it into 356x4. For burst fire weapons each round in the burst is treated as a single shot. For example, lets say a gun fires a 6 round burst. The first 5 rounds will have the PD effect meaning each of those five rounds in the burst will have 2 bullets but the 6th round will not.
If you are going to invest in PD you want to combine coms and spend skill points such that you achieve 50% or more chance of PD activating. I’ll even go as far as saying that anything less than a 90% is a waste of your skill points. But that’s just me.