The boy was treated for a graze to his head after he and another boy found what they thought was a BB gun in woodland close to Wadhurst Gardens, Weston.
Officers at the cordon for the Westwood Nature Reserve. (Image: Lucy Blackmur)
The pair took the gun home and accidentally fired it outdoors once, police said.
Hampshire Constabulary confirmed that the gun was actually a pistol.
LIVE UPDATES: Cordon in place as search continues off Southampton road – live
The force was alerted to the situation just before 5.30pm on Tuesday evening.
Officers responded and were taken to the area where the gun was found by the children.
Parts of the Westwood Nature Reserve have remained cordoned off for three days and they remain at the scene this morning as searches continue in the woodland.
Chief Inspector Chris Douglas, deputy district commander for Southampton, said: “As soon as this incident was reported to us on Tuesday, our teams were swiftly in attendance and have been carrying out thorough enquiries and searches in the area since.
“We know that our scenes will have disrupted people’s normal daily routines, and we want to thank local residents for their patience as we carry out our investigation.
(Image: Lucy Blackmur)
“Thankfully, this discovery has not resulted in more serious injuries, and the boys and their families have acted quickly to report the matter to us and allow us start further enquiries.
“These kinds of firearms are often used to threaten people, and cause serious harm, and we are committed to tackling and disrupting serious criminal activity.
“Our teams remain in the area today, and will be conducting patrols and house to house enquiries, if you do have any concerns or questions, then please do not hesitate to speak with them.”