Police are investigating how a Serbian armoured carrier containing live grenades and machine gun ammunition was allowed into the country.
The carrier is believed to have seen action in the Balkans
The arms were found on an M60 personnel carrier at the Battlefield Live laser games centre, near Bideford in Devon.
Staff cleaning the newly-acquired vehicle found two grenades and about 50 live rounds which were dealt with by bomb disposal experts on Tuesday.
The vehicle is believed to have been used in the 1990s Balkan conflict.
Devon and Cornwall Police are trying to discover how the armoured carrier came into the UK, amid concerns there could be similar vehicles in the country.
Paul Diamond of Battlefield Live said the grenades were found behind two fuel tanks and the ammunition was on the floor.
He said: "I was completely amazed.
''I assumed because of the age of the tank it would have been thoroughly investigated."
He believed it was given up by the Serbs as part of an international aid deal.
Mr Diamond added: "This vehicle obviously saw action in war zone areas and from what we know it was used as a forward observation for artillery over the hills of Sarajevo in the late 1980s."
Mr Diamond bought the carrier from paintball equipment supplier Tony Lewis in Scampton, Lincolnshire, who left it in his yard for two years before selling it.
Mr Lewis told BBC News: "I bought it because it was interesting.
"The inside was full of crisp packets and other rubbish, but there was nothing else that was obvious.
"They had to remove the fuel tanks to find the grenades which is quite difficult, so they could have been there for 20 years.
"You would have to go to extremes to find them."
He said he had bought the vehicle from weapons supplier York Guns, which was unavailable for comment.
The Ministry of Defence said it was unaware of any aid deal involving armoured carriers.
HM Revenue and Customs said it could not comment without details of how the vehicle came into the country.
A spokesman for Devon and Cornwall Police said: ''A controlled explosion was carried out on the hand grenades. The ammunition has been retained by the bomb disposal team.''
The vehicle is now part of Battlefield Live, which is similar to paintball but uses infra-red guns.