。
The thug's pistol was fired.
Literal
Thug [possessive-の] pistol [subject-が] was-fired.
発射された is the passive of 発射する (to fire/discharge). The passive keeps the focus on the event — who pulled the trigger is left unstated. ピストル is a katakana loanword from English. 暴漢 (violent thug/assailant) is a formal compound rarely heard in casual speech.