The hammer is a part of a firearm that is used to strike the percussion cap/primer, or a separate firing pin, to ignite the propellant and fire the projectile. It is so called due to the fact that it resembles a hammer in both form and function. The hammer itself is a metal piece that forcefully rotates about a pivot point.

The percussion cap, percussion primer, or caplock, introduced in the early 1820s, is a type of single-use percussion ignition device for muzzle loader firearm locks enabling them to fire reliably in any weather condition. Its invention gave rise to the caplock mechanism or percussion lock system which used percussion caps struck by the hammer to set off the gunpowder charge in rifles and cap and ball firearms. Any firearm using a caplock mechanism is a percussion gun. Any long gun with a cap-lock mechanism and rifled barrel is a percussion rifle. Cap and ball describes cap-lock firearms discharging a single bore-diameter spherical bullet with each shot.