What i think GS should be is quite simple. You all know the scene from The Matrix where Neo walks through the metal detector, pulls out guns and pwns everyone? You know, pulling out more guns after the previous have been emptied and then using them? This is kind of what i think GunSlinger should be more like. Using 9mms, empty them. Switch to G18s, empty them. Switch to HCs, empty them. Pull out the .500, empty it. Begin the long reloading process. Being able to pull out numerous weapons and shoot a wall of bullets like a Commando with the pup and AK, but not the same.
With the large number of loaded weapons, Gunslinger can pull of a large number of individual kills with a long reload as the weakness. Like Support but not the same.
Being able to kill the stronger specs when the right circumstances appear, like the Support or Commando. Killing the larger specs requires skill, a halfway competent team and the right amount of room.
For instance, GS can take down the scrakes in a similar way to Commando, but with more ammo used. It takes half a mag from a bullpup/AK and half from a SCAR to kill a scrake with about 20-30m between them.
With a revolver as the final weapon he can take down Fleshpounds, but he'd need a long hallway to pull this off and would need luck and help to manage.
The whole idea would be based around versatility and being able to do every job. But not as good as the perks that also do the same job. This could be through using more ammo to accomplish the same thing, or something else; but the point is: yes GS can do your job. No it can't do it as well as you.
This perk could see use in pub games when the team is a wildcard and you don't know what perk to go. Oh, they already have a sharpshooter, i'll go something else.
He's using an M14. dun dun dunn.... "your squad has been wiped out."
If there was a GS on the team this could have been avoided with the perk's versatility.
If you say that Gunslinger shouldn't be added because it does the same thing as other perks, then by this logic Demo shouldn't have been added. It did the same thing as Commando and Firebug; it even contributed to the decline of Firebug's popularity.
If this perk is redundant, then a Zerdic makes Berserker redunant, as does Commando with a katana.