A firearm is generally a rifle, pistol, shotgun, or other portable weapon that expels a projectile (bullet) by use of an explosive action. The definition of a firearm may vary from state to state under state statutes—and under federal statutes—and related items such as a muffler or silencer may be included in the definition of a firearm for purposes of certain criminal offenses (crimes).

In New Hampshire, firearms are regulated under both state and federal laws. The state defines a firearm in a manner consistent with the general understanding of the term, which includes rifles, pistols, shotguns, and other portable weapons that discharge projectiles by means of an explosive action. New Hampshire does not require a license to own a gun, nor does it mandate the registration of firearms. However, certain individuals, such as convicted felons and those deemed mentally unfit, are prohibited from possessing firearms. Additionally, federal law, which also applies in New Hampshire, defines a firearm to include any weapon which will or is designed to or may readily be converted to expel a projectile by the action of an explosive, and includes the frame or receiver of any such weapon, as well as firearm mufflers and silencers. It's important to note that federal law requires background checks for gun purchases from licensed dealers and imposes restrictions on the ownership of firearms by certain individuals, such as convicted felons and individuals with a history of mental illness.