It is not a 21 gun salute, it is 3 volleys of musketry. The firing detail (NOT a firing squad) normally consists of 7 riflemen, but may vary depending on the availability of personnel. The tradition goes back to the days of the Roman Legions, when clapping the hands three times was a way to bid farewell to soldiers leaving for war.
Salutes are fired with cannons. The number of rounds signifies the level of importance of the dignitary being honored, 21-gun salutes being reserved for heads of state. A 50-gun salute is fired on the 4th of July , one round for each state.
Good post, nevertheless, very interesting and impressive. Thanks.