Well regarding the point about liking shotguns, demo, ect, those classes actually add significant different weapon dynamics (shotguns for closequarter crowd control, demo for longer distance crowd control, firebug for softening mobs, commando to pick off most mobs coming from everywhere (along with EBR sharpies) and sharpies helps take out the big ones. Berserker distracts mobs and mele which is completely different from any other class (and medic heals). BUT gunslingers can only take the role of the commando or sharpshooter. It is completely nothing different enough to warrant its own class. Especially if you argue for versatility which either will make it stronger than Commando or sharpshooters, or weaker than either one, the latter resulting in players choosing a commando and a sharpshooter to get better results in higher difficulties rather than go as a "jack of all trades master of none" that does the job of commandos and sharpshooters worse. There's a reason handguns are always backup weapons. Making a perk based around a backup weapon may become a "fun class" but it does the job worse than the classes it's trying to become (unless you make it stronger than either one undermining the use of sharpshooters and commandos). Thus, this is completely redundant. Just make it a small sidearm bonus for every class but much weaker than primary weapons so that it actually becomes what it is: a backup weapon. There really isn't any advantage of having a versatile class does the job of every other class worse. 6 gunslingers won't be as good as a team with one of each class (again, unless gunslighers gets a massive buff that make it primary strong which will result in undermining other classes)
Paragraphs. Use them.
Acually replying, though.
There is one thing you're overlooking.
If you happen to be a commando and has a firebug right next to you, and you see three FPs spawn (in suicidal) right in front of you. What will you do?
The Sharpshooter is nowhere to be seen, and neither you nor the FB posess enough firepower to take them down.
Or, if you're the sharpshooter (and you dislike EBRs and won't use them - and you're not using pistols as well) and 10+ gorefast spawn close to you.
A commando/firebug/support is nowhere to be seen, and you don't posess enough rate of fire to take them down.
In both those cases, you'll die.
And that's because you lack the correct weapon to take those zeds down.
Being a gunslinger means you'll always have a weapon to do the job. Of course, you won't be able to take down single handedly three FPs on suicidal, but I assure you you'll have much higher chances to take them down than a commando would.
A gunslinger adapts to the situation he's facing. No other perk does that. Say what you want, but no other perk can handle every situation he faces with relative ease (except an EBR sharpie, which is why it needs a nerf). And I say relative because you could take down a FP even with a firebug alone on a 6-man suicidal server. But it's incredibly situational, and bogs down to luck more than skill (no other zeds spawn to bother you, environment helps, etc)
This is the gunslingers biggest accomplishment: replacing luck by skill. A skilled gunslinger will be somewhat able to handle everything.
And as said hundreds of times, you won't be able to do it better than the classes meant to do it.
I believe you're not correctly understanding this. You can't compare the gunslinger to a single perk, or compare him to a situation with both the extremes present (Commando + Sharpshooter). Of course a pair of commando + sharpie will fare off better than commando + guns or sharpie+ guns, or guns + guns. However, the gunslinger shines when you lack one of those perks. Teaming up with a firebug will give that player the protection of both a commando and a sharpshooter, though weaker. While FB+Comm will do well against certain types of zeds, it'll fail against others, and likewise for FB+Sharp.