Forensic scientist Pauline McSorley, 55, said she took wet and dry swabs from the right-hand grip plate of a black handgun discovered near a car park in Academy Street, Coatbridge, North Lanarkshire, and found it contained DNA profiles of “at least three people”.
The Scottish Police Services Authority employee said the majority of it, or the “major source”, matched that of Ross Monaghan.
Monaghan is accused of murdering Kevin “Gerbil” Carroll in an Asda car park in Robroyston, Glasgow, on January 13 2010, by repeatedly shooting him.
He is also alleged to have disposed of a revolver, a pistol and ammunition in undergrowth in Academy Street.
Giving evidence at his trial at the High Court in Glasgow, Ms McSorley said there was a “one in a billion” probability of the DNA sample matching someone else’s.
Advocate depute Iain McSporran asked her: “So, in scientific terms, it was a perfect match?”
She replied: “Yes.”
She said that as it was such a small amount, she would be unable to conclude how it got there.
Ms McSorley told the court: “A low-level amount of DNA can be transferred directly or indirectly, so I can’t give you a whole-hearted explanation of how it got there.”
Mr McSporran gave her a number of scenarios, including if Monaghan had handled the gun or if he had been in the same place as it without touching it.
She said: “I would consider them all equally.”
The court heard that Monaghan, 30, provided police with cells from inside his mouth in July 2010.
He denies all the charges against him.