The 12-year-old boy was found at the scene with non-life-threatening injuries and was taken to hospital.
Police said three other boys were seen fleeing the scene — one on bicycle and two on foot.
One suspect, 14, was injured by a police dog while being arrested. Another boy, 12, was taken in without incident.
Police said a third possible suspect is still at large.
Charges have not been laid and the investigation is ongoing. Police have not recovered the firearm.
Anyone who has information is asked to call Regina Police Service at 306-777-6500 or Crime Stoppers at 1-800-222-8477.