The man was walking his dog along Ecton Brook Road in Northampton, when he saw a group of four youths using an air rifle to shoot at a streetlight, by the junction with Mushroom Field Road, and told them to stop.
The incident happened between 7.15pm and 7.30pm on Wednesday, February 27, Northamptonshire Police today confirmed.
After being verbally abused by the group he walked away. As he did so, one of the boys shot at him, with the pellet striking his upper arm causing bruising.
A spokeswoman for Northamptonshire Police said: "The youth who fired the air rifle is described as white, aged 15 to 17, about 5ft 7in and slim. He had short brown hair cut very short at the sides, and wore tracksuit and trainers.
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"He was with three other boys of similar ages, heights and wearing similar clothing. Two were white and one was mixed race."
Anyone who saw the incident, or has information about it, is asked to contact Northamptonshire Police on 101. Information can also be shared anonymously via Crimestoppers on 0800 555111.