In terms of crit-locking monsters, anyone with 190 ASPD and high enough critical rate will be able to do so.
I'm a novice at Gunslingers, but from what I know, pistols have the most utility as it has burst damage, spammability, mobbing capability (somewhat), and disabling, along with chain action to further boost damage and having the most variety with the most card slots.
That said, I have seen shotguns used for PvM as it has better mob killing capabilities and Gatling guns are used for low budget high attack speed builds. Rifles are less often used because any skill used by rifles are also usable by pistols, and Gryphon and other (relatively) obtainable cards kind of dilutes the usefulness of rifles. However, rifles are much easier to build up to high critical rates regardless. And finally, grenade launcher can only attack with grenades, which is suitable for specific situations only, and that's why you don't see it that much.
