A 21 Gun Salute is a Military Honor fired from 21 "Guns" or heavy artiliery or "Canons" NOT Rifles
At military funerals, one often sees three volleys of shots fired in honor of the deceased veteran. This is often mistaken by the laymen as a 21-gun salute, although it is entirely different (in the military, a "gun" is a large-calibered weapon. The three volleys are fired from "rifles," not "guns." Therefore, the three volleys isn't any kind of "gun salute," at all)
Today, the national "21 gun salute" is fired in honor of a national flag, the soverign or chief of state of a foreign nation, a member of a reigning royal family, and the President, ex-President, and President-elect of the United States. It is also fired at noon of the day of the funeral of a President, ex-President, or President-elect, on Washington's Birthday, Presidents Day, and the Fourth of July. On Memorial Day, a salute of 21 minute guns is fired at noon while the flag is flown at half mast. Fifty guns are also fired on all military installations equipped to do so at the close of the day of the funeral of a President, ex-President, or President-elect.