Most of the time, weapons have 2-3 damage bars. The Rookie Machine Gun is the first Weapon to have 1 damage bar, and after the Earth Day Event, the Airsoft SMG is the second weapon with 1 damage, making these two the weakest guns in the game in terms of damage output.
Most Gold Weapons have 4 damage while Cash weapons have either 2 or 3 and sometimes 4. Explosive Weapons have at least 3 bars of damage. Premium Weapons (Event Weapons or limited-time Weapons) mostly have 4 bars of damage.