They warned the public not to approach Robert Stuart Crawley who has gone missing from his home in Chalfont St Peter in Buckinghamshire.
Amersham police believe the youngster is armed with two .22 calibre rifles, two .22 calibre hand guns and ammunition.
Guns were taken when a house in Chalfont St Peter was burgled, said police, but they did not know who stole the guns.
Superintendent Roger Young said: "We have made extensive inquiries in the area and we are conducting a search of areas he is known to frequent. We are using a helicopter, police dogs and some armed officers have been deployed in the area.
"We appeal to anyone who may know where this boy is to contact us immediately and not to approach him. It is essential we trace him as soon as possible. We are most concerned for his safety."
Robert, 5ft 6in, slim, fair-haired, pale with blue eyes, was last seen on Friday at around 9.30am.
Detectives are worried for his and the public's safety because a boy fitting his description fired on a passing Austin Metro in Chalfont St Peter on Saturday at about 3.30pm. The car window was shattered but the driver was uninjured.
Robert was last seen wearing a grey Adidas jumper with three stripes at the bottom, a black bomber jacket, black boots and black jeans.
He had recently become excited by a new interest in guns, police revealed.
A spokesman said: "During our investigation we have found that Robert recently expressed an interest in firearms and possibly joining a gun club."
Mr Young said police involved in the hunt for the boy would shoot if forced.