So after playing quite some last few days, this is how I would personally rank the guns:
S tier - Spitfire, Peacekeeper, Wingman
A tier - R-301, R-99
B tier - Hemlok, Longbow
C tier - G7 Scout, Devotion, Prowler, Triple Take, EVA-8 Auto
D tier - RE-45, Alternator, Flatline
You're better off with your fists tier - Mozambique, P2020
Didn't rank Mastiff and Kraber, I quite like both but due to low ammo I don't pick them up often, especially Kraber.
Spitfire is an absoute beast, imo the best in game due to high TTK in this game. Peacekeeper also when it's consistent. Wingman is insane if you can hit your shots.
R-301 and R-99 are great options for long and close range respectively.