Glossary

Definitions here are technical rather than colloquial. Where a term is argued over, the entry states the definition it uses and why, instead of picking a side quietly.

Ballistic coefficient: A number describing how well a projectile keeps its velocity against air drag, relative to a standard reference shape.
Blowback: An action in which nothing locks the breech: the bolt is held closed only by its own mass and the recoil spring.
Bullpup: A layout with the action and magazine behind the trigger, so a long barrel fits in a short weapon.
Cartridge overall length: The length of the complete loaded round, base to bullet tip.
Case length: The length of the cartridge case alone, from the base to the mouth, not including the bullet.
Closed bolt: A firing cycle in which the bolt is already forward and locked between shots; only the hammer or striker moves when the trigger is pulled.
Cyclic rate: The rate at which an automatic weapon completes its firing cycle, in rounds per minute, if the trigger were held and ammunition never ran out.
Delayed blowback: Blowback with a mechanical disadvantage — rollers, a lever, friction or gas — slowing the bolt's opening without locking it.
En bloc clip: A clip that is loaded into the magazine along with the cartridges and stays there until the last round is fired.
Gas operation: An action driven by propellant gas tapped from the barrel through a port, acting either on a piston or directly on the bolt carrier.
Headspace: The distance from the closed breech face to the surface in the chamber that stops the cartridge going any further forward.
Long recoil: A locked-breech action in which barrel and breech stay locked together for the entire rearward stroke.
Muzzle energy: The kinetic energy of a projectile as it leaves the muzzle: one half the mass times the velocity squared.
Open bolt: A firing cycle in which the bolt is held to the rear between shots and runs forward to chamber and fire in one motion.
Rifling twist: The distance in which the rifling makes one complete turn, and so how fast it spins a bullet.
Rimmed case: A cartridge case with a flange at its base wider than the body, used to headspace the round and to grip it for extraction.
Short recoil: A locked-breech action in which barrel and breech recoil together for a short distance, then separate.
Stripper clip: A metal strip holding several cartridges by their rims so they can be pushed down into a magazine in one motion.
Suppressor: A tube of expansion chambers fitted to a muzzle to slow and cool escaping gas, reducing the noise of the shot.
Trajectory: The curved path of a projectile in flight, and by extension the table or chart describing it.