The fire command, invoked by pressing f, fires a quivered weapon at a square that you select (usually one with a monster). After pressing f and selecting a target to aim at, you can then fire by pressing:
- f (again) or Enter (to fire past the target, potentially hitting whatever's behind it if the first target is missed) or
- . (to fire directly at the target's tile, guaranteeing that the projectile will not travel past the tile you're aiming at even if it misses all targets).