Terry Collins said the pistol was his most exciting discovery
Lucky Tony Collins was at the seaside on a cold afternoon when he came across the historic weapon – that was still largely intact.
Mr Collins, who has been metal detecting on Jersey beaches for four years, described it as his most exciting discovery ever.
And experts have hailed it as a "very special object" and hope it will go on display at a museum.
Mr Collins, who is a member of the Jersey Metal Detecting Society, said: "There was nobody else on the beach and when I pulled it out of the sand at first I wasn't sure what it was.
"But when I knocked some of the encrustation off I saw the barrel and immediately realised it was significant."
"I wasn't expecting to find anything in that particular location."
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The rusting pistol is from the 17th Century and was used during the English Civil War, according to experts.
We will probably never know how it ended up where it was
It was found in St Aubin's Bay in Jersey and has an unusual complex wheel-lock mechanism – typically found on pistols from the 1600s.
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Charity Jersey Heritage have said the find is unique to its island.
Conservator Neil Mahrer said: "We find quite a lot of weapons from later periods but this is the first one I have seen from this age, and with the metal still intact. It is a very special object."
The rusty pistol was used in the English Civil War according to experts
Mr Mahrer has begun the conservation process of separating the metal parts from the wood.
He will then dry them out and conserve them before rebuilding the weapon.
It's thought that so much of the original material has been preserved because the pistol was lying in black sand – which has very low oxygen content.
X-rays will be used to uncover more of the weapon's secrets.
When it has been cleaned and stabilised, Mr Collins hopes the pistol will go on display.
He added: "We will probably never know how it ended up where it was.
Terry had been metal detecting on St Aubin's Bay for four years before the discovery
"But the most likely scenario that an officer who was boarding a ship dropped it into the sea.
"In its day it would have been a very expensive item so if it has just been dropped on the sand, the owner would have picked it up."