How Guns Work A gun is a weapon that uses the force of an explosive propellant to project a Guns or firearms are classified by the diameter of the barrel opening. This is known as the calibre of the gun. Anything with a calibre up to and including . 60 calibre(0.6 inches) is known as a firearm. The precise origin of the gun is unknown, although they were in use by the early 14th century and were common place in Europe by mid-century. These early guns were nothing more than large calibre cylinders of wrought iron or cast bronze, closed at one end and loaded by placing gunpowder and projectile in the muzzle, or open end. Nowadays firearms are a little more sophisticated. Howeve