As of Feb. 22, 2010, federal law allows people who can legally possess firearms under applicable federal, state, and local laws, to legally possess firearms in national park units in that state. This generally includes campgrounds. In other words, state law applies. Note that federal law does prohibit firearms in certain facilities in the parks, typically visitor centers and other park buildings and often on ranger-guided tours. Note also that discharging the weapon for any reason is prohibited. In a worst-case scenario, a citation (or even arrest) might be the least of your concerns.