Some numbers from a 2011 NYPD report (only New York City):

Situations where firearms were discharged

36 - Adversarial Conflict

36 - Animal Attack

15 - Unintentional Discharge

02 - Unauthorized Use

03 - Suicide Attempt

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124 - Total officers firing

416 - Total shots fired

003 - Total officers shot and injured by subject

001 - Total officers shot and killed by subject

019 - Total subjects shot and injured by officer

009 - Total subjects shot and killed by officer

http://www.nyc.gov/html/nypd/downloads/pdf/analysis_and_plan...