Tony Collins made the discovery in St Aubin's Bay, Jersey.
The English Civil War era relic has an unusual complex wheel-lock mechanism, found on pistols typical of the early 1600s.
The weapon has retained much of its original material, thought to be because it was found in black sand, which has a low oxygen content.
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Mr Collins, who has been metal detecting for four years, once found a well-preserved (and still live) shrapnel ball from 1805, but he says this latest discovery is his most exciting.
"I wasn't expecting to find anything in that particular location," he said.
"There was nobody else on the beach and when I pulled it out of the sand and 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."
Neil Mahrer, conservator at Jersey Heritage, confirmed the pistol was important.
He 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."
As part of an intricate process to restore the pistol, it may be X-rayed.
Mr Collins hopes the item will go on display when the process is complete, which could take more than a year.
A rare 17th Century officer's pistol has been found by a metal detectorist scanning a beach at low tide.