they were used in anti-submarine warfare during the First World War and were the subject of a 1921 United States patent (US Patent No. 1365869). Types Powder actuated tools can be variously classified: Direct acting (the charge acts directly on the head of the nail or high velocity), or indirect (using an intermediate piston or low velocity) Single-shot, or magazine-fed Automatic or manual piston cycling Automatic or manual feed of the charges Energy sources Powder-actuated tools are powered by specially-designed blank firearm cartridges, also informally called "loads", "boosters", "rounds", or "charges". In many cases, the charges are ordinary firearm cartridges with modified casings, and the bullets omitted. The .22 Short, developed by Smith & Wesson, is common. These charges may be hand-fed (single-shot), or manufactured and distributed on a plastic carrier strip. Color coding The three single-shot strengths or colors typically sold to the general public are brown, green, and yellow in brass-colored casings. Not all powder-actuated tools are rated for high-capacity charges—the strongest charge (nickel-purple at ), for example, is dangerous in a tool not rated for the high pressures it generates. The table above is for a slug from a test device. Safety and regulation As with their air-actuated cousins, powder-actuated guns have a muzzle safety interlock. If the muzzle is not pressed against a surface with sufficient force, the firing pin is blocked and cannot reach the load to fire it. This helps ensure that the gun does not discharge in an unsafe manner, causing the nail to become an unrestrained projectile. Due to their potential for causing personal injury, OSHA regulations in the US require certification specific to the tool being used before any person is permitted to rent or use powder-actuated equipment. Most manufacturers of powder-actuated nail guns offer training and certification, some with free online testing. In addition, special instruction is necessary if the prospective user is unable to distinguish colors used in the color code system that identifies proper power levels. Most certifications are accepted for life; however, in California they must be renewed every three years. The ownership and use of these tools is regulated in Australia. The owner has to register the tool, and an operator of one of these tools is required to have a license and to have undergone training in their use. These laws are in keeping with Australia's extremely strict firearm laws. See also Pneumatic tool Power