I’m trying to make, like, a pirate gunslinger type build. I started with Sheriff and I’m thinking of going Corsair. I basically just want to roleplay a pirate, but I’m EXTREMELY new to this game. I’m wondering if this is even viable and what else should I go to make this build viable 
Any help would be appreciated for a brand new player <3
You should build the way you imagine first, until you have enough experience about the contents of the game, I believe that any build are able to complete all the main story line quests, except full support maybe. You can change your class once you have accumulated enough points, don’t worry if you messed up.
you can reset build , for a new player SR or BM is the easiest way for scout leveling and quest , chose shinobi if you want naruto run to area 51