The armed response officer was taking part in a training exercise at a military range near Carnoustie, Angus, in April, when his weapon was discharged by accident.
The weapon fired a bullet which hit a brick wall, sending debris flying towards one of his colleagues.
Scotland's independent Police Investigations and Review Commissioner, or PIRC, Professor John McNeill, yesterday said the discharge had been negligent and preventable.
However, it is understood the officer hit by brick debris had only been slightly injured.
The shooting took place at Barry Buddon firing range on the morning of April 28 and, like all firearms incidents, was investigated by Mr McNeill's team.
The Accuracy International 7.62 rifle used is of standard military and police issue and is used for sharp shooting.
Mr McNeill said the accident would never have happened if officers had stuck to tough rules on handling such weapons.
Professor McNeill said: "An inspection of the weapon by a firearms specialist found no faults.
"The only way that the incident as described could be duplicated was by pulling and holding the trigger to the rear and at the same time disengaging the safety catch.
"It follows therefore that the discharge was negligent and preventable."
He added: "Police Scotland firearms practice and procedures in respect of firearms safety are stringent.
"However, as a result of this incident, several recommendations have been made by Armed Policing Training to strengthen these further."
Chief Superintendent Elaine Ferguson of Police Scotland's Operational Support Division said: "Police Scotland note the findings of this report, in particular the recognition of the stringent practices which are in place to prevent such occurrences.
"A number of local measures have already been undertaken to enforce these and ensure their full adherence by staff."
Asked if the officer who was responsible for the weapon was to be disciplined, a force spokesman said: "Internal inquiries are ongoing."
The PIRC is also investigating an incident in Dumfries in May this year.
Another armed response officer accidently tasered his or her finger.
The officer was putting the weapon in to a leg holster when it accidentally discharged its electric barbs.
This matter is also subject to a parallel internal Police Scotland review.