Pistol/revolver. 3 upgrades to pick.
1: increase ammo capacity. From 6 to 24
2: fire bullets. Each shot can cause burning damage.
3: barrel upgrade. Increases damage

Shotgun. 2 variants to choose from. Pump action or double barrel

Pump upgrades: electric shot. When charged the bullet causes electric damage. 

Turns the pump into automatic shotgun.

Double barrel: increase capacity. Installs two rotating chambers that hold 3 each. So the shotgun can hold 6 shells total.

Sawn off barrel: increases damage. (Somehow)

Crossbow. Can add a scope to it. Speed loader box and the bow can be upgraded to have increase chance to collect the bolt again.

Machine gun: improved frame to decrease recoil. Barrel upgrade to improve accuracy. Spin upgrade spins the bullet and increases damage.

Grenade launcher: blast immunity. (No clue how those actually work). Air pressure launcher increases range.

Spear gun: rotating drum allows speed loading and ammo capacity increased and increase fire rate. So 3 upgrades in one yay. A retractable scope