Gunslinger Master by itself does nothing?

I'm a newb.

Am I correct that having stacks of Onslaught by itself grants no benefit? For example, if the only Perk card I have equipped that mentions Onslaught is Gunslinger Master, and I don't have any legendary mods that mention Onslaught, then those stacks of Onslaught are just pointless?

I've tried shooting things with and without the card equipped. The damage is different but not significantly so. I think that the difference I'm seeing is based on other range variables or something.