A police officer fired a confiscated gun at a police station, prompting an investigation.
Greater Manchester Police said the blank-firing pistol was discharged by a police constable while it was being processed at Leigh Police Station on Tuesday 15 December.
An investigation was launched and the officer was "taken off front-line duty for his shift", the force said.
A senior officer "strongly advised him of his conduct", a spokesman added.
Further details of the circumstances have not been revealed although nobody was hurt and no damage was caused, police said.
The force spokesman said: "A referral to the force's Professional Standards Branch was made and a decision was taken that the matter would be dealt with at a district level where a senior officer strongly advised him of his conduct."