You begin the game proper, following Walter’s tutorial mission, with the Flaming Compensator, High Fidelity, and Dirty Harry—a well-rounded arsenal for use against the OD or even Scabs. Fizzco robots, on the other hand, will give you trouble until you’re able to accumulate something more effective, such as The Shocker.
Assuming that you stick entirely to the single-player campaign and never touch Chaos Squad, you will gradually unlock almost every weapon in the game by progressing through the story. By the time you clear “Concertpocalypse,” you should have every gun either in your inventory or available for purchase via Two Hat Jack (with the exception of the ProPain Launcher).
Many of the later guns are prohibitively expensive, starting with the Fizzbot Rifle, which is meant as an incentive to play Chaos Squad, where you can earn them for “free.” There is a carefully managed amount of Overcharge available in the single-player game via Mission, Quest, and Challenge rewards. If you clear most of the Missions, Quests, and Challenges, you should have just enough Overcharge to buy all the available weapons as long as you don’t spend too much on Jack’s Collectible maps.
If you do play Chaos Squad, you can get a number of rewards from the “spinners” at the end of a successful match. These rewards can include cash, Overcharge, clothing and other cosmetic accessories, guns that you haven’t unlocked yet, the ProPain Launcher, temporary power-ups for various guns, and level-2 versions of Amps. Chaos Squad is the only way to get the ProPain Launcher, as well as several other cosmetic items.