Taunton Magistrates? Court heard that Sir Benjamin Slade, whose ancestors fought in the Crimean War and had links to Horatio Nelson, had legally acquired a Fabarm semi-auto before 1987 but had neglected to put it on his certificate.
Slade claimed he had spoken to his police firearms officer about it and had thought it was on his certificate.
He was also found guilty of leaving an AYA shotgun outside his gun cabinet in a locked room.
In a statement to the court he said the gun had been taken out in the morning to deal with a fox that was eating his geese.
Peter Glenser, defending, said: ?For more than half a century he has held and used shotguns and firearms legitimately and without incident. Now he is unlikely to get his guns back.?
Police also accused Sir Benjamin of an offence relating to an antique Purdey shotgun that had come into the family in 1859 and was displayed on a wall in his home with the firing pin removed, along with 85 other decommissioned guns.
The charge was later dropped, as was another relating to a failure to secure his gun cabinet to the wall of his house.
Jamie Foster of Clarke Willmott LLP, Sir Benjamin?s solicitor, said: ?Sir Benjamin accepts that he should have been more careful in this case to ensure that everything was in order. He is very relieved to put this case behind him.?