Who is the actor in BLOW who puts gun Johnny Depps mouth?

The actor who puts a gun in Johnny Depp's mouth in the movie *Blow* is Ethan Suplee. He plays the character of "Tuna," a drug dealer who becomes George Jung's (Johnny Depp's character) partner in crime.

The scene you're referring to occurs when Tuna, increasingly paranoid and unstable, forces George to take a large amount of cocaine. He holds the gun to George's mouth as a way to intimidate him and force him to comply.