may be loaded, rounds may be chambered and firearms may be fired. When everyone on the firing line agrees for the line to go cold, magazines must be removed, firearms must be unloaded, actions opened, and all the firearms must be placed on tables. Only upon confirmation does the line actually go cold. Firearms may not be loaded or even touched when the line is cold. Outdoor shooting ranges are usually backed by a sandbagged barrier or specially-designed funnel-shaped traps to prevent bullets from ricocheting back at the shooters. Many older outdoor ranges only use an earthen berm, which is often composed of sand, and called the impact berm. Most outdoor ranges restrict the maximum caliber size, or have separate ranges devoted to use with rifles firing heavy caliber cartridges. Many outdoor ranges do permit the use of automatic weapons on specially adapted Class III weapons ranges. The largest problem currently with outdoor ranges is the lack of sufficient area behind and beside the impact berms. This area, usually called the Surface Danger Zone (SDZ), is a fan-shaped area that extends lengthwise to the ultimate ballistic distance of the round fired. Outdoor ranges often use baffles to contain fired rounds within the range, and, with proper design can reduce the surface danger zone area. Guidelines for surface danger zones vary widely. The National Rifle Association maintains guidelines for range design, but often the U.S. military range design guidelines are more prescriptive, specific and easier to use. Targets using Tannerite are sometimes used for indicating long-range markmanship accuracy at high power rifle and full-auto outdoor ranges. Tannerite is a binary explosive that is not exploded unless hit with high-velocity rounds. Most pistol rounds will not set it off. Indoor ranges An estimated 16,000 to 18,000 indoor firing ranges are in operation in the United States. Indoor ranges differ in construction, depending on the type of weapons to be fired: On a rifle range, shooters typically fire pistol caliber carbines at targets 100 yards (91 meters) distant. The common practice is to follow rules much like those required on outdoor ranges. On a handgun range, shooters fire semi-automatic pistols, revolvers, and (sometimes) submachine guns at stationary targets along, or at the end of, a 15-100 yard (14-91 meter) long lane. Paper or cardboard targets are hung from a dummy or target holder positioned on a slide; this arrangement allows the shooter to