Retailers are being warned by Devon and Cornwall Police not to sell pellet guns to children after a 12-year-old bought one at a gift shop in Torquay.
Tracy Russel said she was so surprised when her son, Daniel, brought the pistol home she called police.
Police spoke to the shop and said the owner learned a "very severe lesson".
The force said that misuse of the guns, which fire plastic pellets, could result in someone being hurt and armed police being called out.
Ms Russel said: "He came back with this gun after he walked in on his own, picked it up, brought it to the counter and they sold it to him.
"I contacted the police because I was not very impressed and because it said 18 and upwards on the box."
Police said it was not just the buying of such guns by someone so young that had them concerned.
Pc Mike Bradley, of Devon and Cornwall Police, said: "It's also because the fact that these guns can hurt people.
"Also, in low lighting, and somebody reports seeing one to the police, you could have an armed response officer attending that incident thinking it's a real gun."