Two further charges of possession of an illegal firearm against the accused were dismissed by the court.
The prosecution withdrew the charges during a final submission following a one day trial last month.
The weapon, an assault rifle, had gone missing from the exhibit room sometime between 2013 and 2015.
The police say the rifle was found in a man's vehicle during a raid in 2016.
The man, Lonino Aumua Brown, had told the court the weapon was sold to him by Mr Lesa.