Gun usage in hunting is typically regulated by game category, area within the state, and time period. Regulations for big-game hunting often specify a minimum caliber or muzzle energy for firearms. The use of rifles is often banned for safety reasons in areas with high population densities or limited topographic relief. Regulations may also limit or ban the use of lead in ammunition because of environmental concerns. Specific seasons for bow hunting or muzzle-loading black-powder guns are often established to limit competition with hunters using more effective weapons.