It happened Monday afternoon in the 600 block of Russell Avenue in the city's Moxham neighborhood.
Johnstown Police Captain Andy Frear said the boy and his friends found the gun in the bushes.
"The bullet grazed his arm," said Frear. "We're lucky that's all that happened to him."
The serial numbers were scratched off of the gun; police will continue to search for the owner.
Anyone with information is asked to call 1-800-281-1680.