The term "gun pendulum" refers to a device used to measure the muzzle velocity of firearms by measuring the swing of a pendulum before and after firing a projectile. Synonyms for gun pendulum include ballistic pendulum, bullet pendulum, and recoil pendulum. These devices were commonly used in the 19th and early 20th century for testing and improving the accuracy of firearms. While they are no longer the primary means of measuring muzzle velocity, they remain an important part of the history and development of firearms technology. Today, modern firearms are measured using electronic sensors or optical methods, but the gun pendulum will always hold a special place in the evolution of ballistics and firearm science.