Federally Prohibited Places
Firearms, loaded or unloaded, concealed or openly carried, are prohibited in the following places
irearms are banned in "a building or part thereof owned or leased by the Federal Government, where Federal employees are regularly present for the purpose of performing their official duties" and includes essentially all parts of federal court facilities (18 USC § 930). 'No weapons' signs must be posted at federal facilities in order for someone to be convicted (but you may be arrested).
Interstate Transportation Protection 18 USC § 926A
This section was intended to protect innocent passage of travelers who might have a firearm prohibited by local jurisdiction. For instance, a resident of a frontier state driving through California with an 'assault weapon' to another free state. The trip must be from one free state to another, the firearm unloaded, and neither the firearm nor any ammunition is readily accessible from the passenger compartment. In the case of a vehicle without a compartment separate from the driver’s compartment the firearm or ammunition shall be contained in a locked container other than the glove compartment or console.
Airports/Aircraft 49 CFR 1540.111 and 49 USC § 46505
Under federal law (local laws may differ), firearms are prohibited beyond the TSA checkpoints. You also cannot attempt to enter a TSA checkpoint while armed, even by mistake. Check your bags! Even loose ammunition, empty magazines, spent cases, and miscellaneous gun parts have caused problems and prosecutions for travelers. Learn more about the TSA regulations.
Self-defense firearms are generally prohibited on base. Almost uniformly, civilians may not possess firearms on base. Service-members may be required to store their weapons as their commanding officer directs. Check individual base and service branch regulations. Additionally, army regulations are here and the oft-mentioned Bush-era directive is here (reaffirmed in 2011).
Section 512, Credit CARD Act of 2009 (and 54 USC § 104906) changed federal law to make park firearm regulations reflect state law. Typically, the only restrictions on firearm carry are state and local laws. If it is legal elsewhere in the state, it is legal in the park and sections that conflict with state law regarding carrying and possessing firearms (but not shooting bans) do not apply. So though on National Park Service lands (National Parks, Monuments, etc.) carrying a firearm or possessing loaded firearms are prohibited in vehicles (unless one has a special park permit, usually for hunting), this does not apply if the state allows open and/or concealed carry, or loaded/unloaded firearms in vehicles.
The park buildings (visitor centers, offices, etc.) are still federal facilities and off-limits to firearms. Discharge of firearms, except when lawfully hunting, is generally prohibited. Discharge of firearms, except when lawfully hunting, is generally prohibited. NPS pamphlet here. The same applies for National Wildlife Refuges, 16 USC § 1a–7b.
Who cannot possess a firearm?
Federal Prohibited Persons
The following are federally prohibited persons: