The Three-Volley Salute vs. the 21-Gun Salute
The rifle party firing at a military funeral is not the same thing as a 21-gun salute — different weapons, different occasion, and the confusion is old enough to have its own explainer at Arlington.
How it actually works
In military terminology a "gun" salute means artillery — cannon or crew-served guns — and the 21-gun salute is reserved for the President, ex-Presidents and the President-elect, foreign heads of state, and the national flag. The rifle party at a service member's funeral fires the three-volley salute: a firing detail of riflemen (traditionally seven, though the number varies) firing three blank volleys in unison. The custom traces back to an old battlefield practice — combat would pause so each side could clear its dead, and volleys signaled the fighting was resuming.
The three-volley salute gets called a "21-gun salute" constantly, and the mix-up is understandable: seven riflemen firing three volleys sounds, to an untrained ear, like a lot of shots stacking up toward twenty-one. But guns and rifles are different weapons with different ceremonial rules, and the two customs are not interchangeable, no matter how often the count gets confused. The three-volley salute is also not a countable ratio of shots-per-person — it is a fixed three volleys from the detail, regardless of how many rifles are firing.
Three-volley custom traces to a battlefield cease-fire practice for clearing the dead; the 21-gun salute is a separate, older maritime and artillery honors tradition reserved for heads of state.