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 muz