First of all - learn his gun's passives.
Go to youtube and just get a mental note on what each gun does, it's easy to remember by color - still it will save you from going "oh shit, why am i rooted from his basic attack" or "Oh shit, why is he floating towards me with mini Lucian ult".
Besides that, he's immobile and squishy with almost 0 self peel, and he HAS to play around his gun's ammo system so at some points he will be weaker than others.
tl;dr about his guns:
Green / Sniper - Gives him cait's AA range, Q ability shoots a linear skillshot that if connects with an enemy allows him to AA him again once.
Red / Pistol - Gives him healing on AA, Q ability gives him MS and auto targets nearest target, kinda like Lucian's ult
Purple / Rocket launcher - Gives slow on AA, Q ability snares enemies hit by his AA.
Blue / Flamethrower - Gives a pseudo titanic hydra passive - enemies hit by his AA will splash in a cone behind them, Q ability sprays the area in a cone infront of him.
White / Chakram - Boomerang weapons, He throws it and cant attack until it's back in his hand (meaning the closer you are the faster he attacks), also the more time he attacks the more passive damage he gains on you, Q ability places a turrret.
His ulti is an AoE skillshot that explodes on enemy contact, the AoE is effected by the gun he has equiped (Sniper one lets him AA anyone hit by ult, Rocket launcher him lets him snare anyone hit by his ult ect.... )