I should add, my experience with non frequency specific +offset DC (zappers) is theyre very chemistry dependent. Its almost like they just ramp up your immune cells and send them on a suicide bombing run. Against giant worms they appear to only work occasionally after the body has had a chance to replace the suicided cells.
In between they don't do much. It' like you're left with non working chemistry (maybe non ionised or oxidised coppper..) until the body recovers.
Chemistry appears to be the thing that define the frequency accuracy required for kills for me. Worse chemistry, more accuracy required.