The Gatling gun is one of the best known early rapid-fire weapons and a forerunner of the modern machine gun

Machine gun

A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute....

. It is well known for its use by the Union

Union (American Civil War)

During the American Civil War, the Union was a name used to refer to the federal government of the United States, which was supported by the twenty free states and five border slave states. It was opposed by 11 southern slave states that had declared a secession to join together to form the...

 forces

Union Army

The Union Army was the land force that fought for the Union during the American Civil War. It was also known as the Federal Army, the U.S. Army, the Northern Army and the National Army...

 during the American Civil War

American Civil War

The American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...

 in the 1860s, which was the first time it was employed in combat. Later it was also famously used in the assault on San Juan Hill

Battle of San Juan Hill

The Battle of San Juan Hill , also known as the battle for the San Juan Heights, was a decisive battle of the Spanish-American War. The San Juan heights was a north-south running elevation about two kilometers east of Santiago de Cuba. The names San Juan Hill and Kettle Hill were names given by the...

 during the Spanish-American War

Spanish-American War

The Spanish–American War was a conflict in 1898 between Spain and the United States, effectively the result of American intervention in the ongoing Cuban War of Independence...

.

The Gatling gun's operation centered on a cyclic multi-barrel design which facilitated cooling and synchronized the firing/reloading sequence.