This game is ridiculously fun. I played it a bunch on my own, but then decided to look up the "best gun" to go for. Everyone agrees on the trigger, barrel, and clip, but there weren't any guides beyond that. I made this, which helped me get to level 135 easily.
For anyone looking for a winning combination, I personally recommend 5x Burst Trigger, Quad Barrel, Arrows, and Bullet Canister. People don't usually bring up arrows, but I think that they're the best bullets to use with this combo because you can pick them up. You can use whatever stock and grip you prefer, but my advice is to use at least one item from the Bee set since they can hit things through walls, and then something to buff your speed.