Just out of prison, ex-con Ugo Piazza meets his former employer, a psychopathic gangster Rocco who enjoys sick violence and torture. Both the gangsters and the police believe Ugo has hidden $300,000 that should have gone to an American drug syndicate boss. Caliber 9 is the first part in Di Leo's "Milieu" Trilogy of poliziotteschi films. It was followed by "The Italian Connection" (1972) and "The Boss" (1973).
Unfortunately the movie Caliber 9 is not yet available on Disney Plus.
| Directing | Fernando Di Leo | Director |
| Writing | Fernando Di Leo | Story |
| Production | Vincenzo Salviani | Unit Manager |
| Writing | Fernando Di Leo | Screenplay |
| Camera | Claudio Morabito | Camera Operator |
| Sound | Goffredo Salvatori | Sound |
| Production | Armando Novelli | Line Producer |
| Directing | Vivalda Vigorelli | Script Supervisor |
| Camera | Enrico Biribicchi | Assistant Camera |
| Directing | Franco Lo Cascio | First Assistant Director |
| Production | Ermanno Curti | Producer |
| Production | Lanfranco Ceccarelli | Production Manager |
| Writing | Giorgio Scerbanenco | Novel |
| Art | Francesco Cuppini | Production Design |
| Production | Luciano Appignani | Production Secretary |
| Editing | Amedeo Giomini | Editor |
| Costume & Make-Up | Antonio Mura | Makeup Artist |
| Camera | Franco Villa | Director of Photography |
| Sound | Luis Bacalov | Original Music Composer |
| Costume & Make-Up | Francesco Cuppini | Costume Design |