We are not identifying him by name because he said he wants to protect his family.
His neighbors are expressing relief and shock about the discovery made Wednesday morning.
"Anybody could have picked it up," one neighbor, Caryl Blevins, told WHIO-TV's Sean Cudahy. "That's just frightening," she said.
Officers Scott Fero and Lindsay Warner of the Dayton Police Department said in an incident report the man flagged them down about 9:15 a.m. to report where he said he found the long rifle. He told them he put it in his car because he feared neighborhood children might find it and hurt themselves.
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The officers, in the report, identified the weapon as a 12-gauge Maverick by Mossberg Model 88 shotgun that had a fore grip, rail light and optic sights. There were five shotgun shells loaded in the weapon, which they were able to unload, and six more shells in holders attached to the side.
One of the officers checked the serial number through the department's records section, and found it had not been reported stolen.
The weapon and its 11 shells are now in the police department's property room, according to a report.
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