The AR-15 rifle was locked to the side of a police motorcycle during an anti-drug assembly held at Newman Elementary on Oct. 23.
Three children received minor injuries. Two of the victims were allegedly struck by metal debris that came from the bullet disintegrating as it hit a metal place where the barrel rests against the weapons mount. A third was examined at the school and released to parents.
The family of the student who suffered ringing in his ears has accepted a $5,000 settlement.
During the later investigation, Chino Police Chief Miles Pruitt said policy was not followed prior to the event. Department rules allegedly state the rifle chamber should be empty when it is mounted.
The Associated Press contributed to this report.