Paul Clarke, 27, was found guilty of possessing a firearm at Guildford Crown Court on Tuesday – after finding the gun and handing it personally to police officers on March 20 this year.
The jury took 20 minutes to make its conviction, and Mr Clarke now faces a minimum of five year's imprisonment for handing in the weapon.
The court heard how Mr Clarke was on the balcony of his home in Nailsworth Crescent, Merstham, when he spotted a black bin liner at the bottom of his garden.
In his statement, he said: "I took it indoors and inside found a shorn-off shotgun and two cartridges. I didn't know what to do, so the next morning I rang the Chief Superintendent, Adrian Harper, and asked if I could pop in and see him. At the police station, I took the gun out of the bag and placed it on the table so it was pointing towards the wall."
Mr Clarke was then arrested immediately for possession of a firearm at Reigate police station, and taken to the cells.
Prosecuting, Brian Stalk, explained to the jury that possession of a firearm was a "strict liability" charge – therefore Mr Clarke's allegedly honest intent was irrelevant. Just by having the gun in his possession he was guilty of the charge, and has no defence in law against it, he added.
Judge Christopher Critchlow said: "This is an unusual case, but in law there is no dispute that Mr Clarke has no defence to this charge. The intention of anybody possessing a firearm is irrelevant."