Christopher Sacco reeled in the weapon in a plastic bag as he fished in the waters off Fort De Soto Park about a week ago.
Mr Sacco called the Pinellas County Sheriff’s Office to report the gun and officers came to take possession of the weapon and placed it in property and evidence, a spokesperson for the Sheriff’s Office said.
The weapon was entered into the Florida Crime Information Center and the National Crime Information Center.
“You’ve got be kidding me, I found a gun” Mr Sacco 35, said to his friend.
“He looked at me like, ‘Are you serious,’” Sacco said. “I’m like bro, I found a freaking gun.”
Josh Benson, a WFLA-TV reporter, confirmed on Twitter on Tuesday that Pinellas County Sheriff’s Office has the gun in evidence.
“GUN. Checked in with the man who found the gun in #FortDeSoto more than a week ago. Says he only took one pic and turned gun over to Pinellas County authorities. Sheriff’s Office tells me they have it in evidence and there’s been no update just yet,” he tweeted.
Fort De Soto Park was one of the locations where Dog the Bounty Hunter, whose real name is Duane Chapman, searched for Laundrie before his remains were found at the Carlton Reserve.
The remains were later identified by dental records but a cause of death has not yet been released by officials.
Mr Sacco said that the area was unusually quiet at the time he found the weapon.
“Usually there’s a whole bunch of people. What if some little kid found it?” He said.
The deputy who responded to the report called the situation “a first”, added Mr Sacco. Authorities have not made any indication that the gun is related to the Laundrie case.
In September Laundrie was named by police as a person of interest in Gabby Petito’s disappearance.
A federal arrest warrant was also issued in Wyoming for his arrest, charging him with unauthorised use of a Capitol One bank debit card between 30 August and 1 September.
Petito, 22, and Laundrie, 23, had spent the summer travelling across the western US in her white van, regularly posting pictures and videos of their “van life” adventure to social media.
On 12 August police in Moab, Utah, encountered the couple after a passerby reported a physical altercation between them.
But Petito’s postings stopped suddenly in August and Laundrie returned to his family’s home in Florida without her on 1 September.
When Petito’s family were unable to contact her they reported her missing 10 days later, as Laundrie refused to cooperate with investigators.
He then went missing himself, with his family saying they believed he had gone to the nature reserve, where his remains were eventually found.
Petito’s remains were discovered in a remote Wyoming national forest on 19 September, where they had last been seen together.
Several days after that, Teton County Coroner Dr Brent Blue ruled her death a homicide and announced she had died by manual strangulation.
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