At about 6:50 p.m., the CHP received a report of a possible shooting on eastbound state Highway 4 just east of Willow Pass Road.
When officers arrived, the victim told them that he had been standing on the side of the highway waiting for a tow truck when he heard a pop and felt something hit his arm, Maguire said.
When he looked down, he saw blood coming from a small hole near his right elbow.
The CHP searched the area by helicopter, but didn't find anything out of the ordinary.
Officers also talked to people at a nearby shooting range and determined that nobody had been shooting in the direction of the highway.
The victim was taken to Kaiser Permanente Antioch Medical Center, where doctors found something spherical in his elbow, but didn't remove it. According to Maguire, doctors said removing the object would be riskier than leaving it in place.
The man is expected to make a full recovery.
Although some reports have speculated that the object in the man's arm was a BB gun pellet, Maguire said investigators have not identified the object and haven't found any evidence that a BB gun was fired.