How many letters does the top answer for “a projectile that is fired from a gun” have?

The most common answer is bullet (6 letters).

Crossword clue: a projectile that is fired from a gun

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

bullet 6 letters wn-def

a projectile that is fired from a gun

slug 4 letters wn-def

strike heavily, especially with the fist or a bat

projectiles to be fired from a gun 100% similar → ammo, ammunition