Cap guns are illegal.
Background information (if available):
Picture thanks to: skand
Explosives and Fireworks
§ 11-13-4 Toy canes or devices for firing blanks. No person or persons shall sell, expose for sale, possess with intent to sell or use, or discharge or use, within this state, any repeating toy cane for discharging any explosive, any toy pistol, or any other toy device designed or used for the discharge of blank cartridges.