The DNA of murder accused Ross Monaghan was found on one of the guns used to shoot gangland figure Kevin 'Gerbil' Carroll to death, his trial has heard.
Forensic scientist Pauline McSorley told jurors the chance of the DNA belonging to anyone other than Mr Monaghan was one in a billion.
But the High Court in Glasgow heard the DNA of a laboratory technician was also found on the weapon.
Mr Monaghan denies murdering Mr Carroll in an Asda car park in January 2010.
The court heard that the gun - a black self-loading pistol - was found alongside a revolver behind Coatbridge Library on 26 January 2010.
Mr Carroll, 29, had been shot dead 13 days earlier at an Asda car park in the Robroyston area of Glasgow.
Giving evidence to the trial, Ms McSorley told advocate depute Iain McSporran, prosecuting, that the gun was swabbed and they found DNA from "at least three people."
She said the major source of the DNA was 30-year-old Mr Monaghan, and described it as a "perfect match".
Under cross-examination by defence QC Derek Ogg, she admitted that one of the other sources of DNA came from a laboratory worker. She agreed that the worker had not been working anywhere near the gun.
Ms McSorley said it was thought that DNA from the female worker had been on the outside of a test tube that had been washed into the sample.
It was an example of "secondary transfer" of DNA, she said.
Referring to the discovery of the guns by a group of gardeners, who opened up the plastic bags and touched the weapons, Mr Ogg said: "It's horrendous that should have happened from a scientific point of view."
Ms McSorley replied: "Certainly, it's not ideal conditions."
The court later heard that Mr Carroll died from gunshot wounds to his head and chest.
The evidence came in a joint minute of agreement read to the jury.
Later firearms expert Ronald Withers, 67, said that Mr Carroll was hit with a hail of 13 bullets as he sat in the back of his black Audi A3.
Another bullet was fired through the front of the car, but missed Mr Carroll.
Mr Withers said Mr Carroll may have tried to protect himself by raising his hands to his head and also picking up a manual lying in the back seat of the car.
Mr Withers told the jury that Mr Carroll was shot through the middle of the forehead and twice in the jaw.
These bullets first entered his wrists and fingers as he tried to protect himself. A fourth bullet was found lodged in the car manual.
Mr Withers said that he believed that the two gunmen stood on either side of the black Audi and fired into the back seat.
He said that one shot which he believed was either the first shot or the last shot was fired through the front windscreen, but did not hit Mr Carroll.
The jury was told that five bullets were fired from the silver revolver and all hit Mr Carroll.
He was hit by eight bullets from the other gun - a black self-loading pistol - which was fired from the driver's side.
Mr Withers said he was present when the car containing Mr Carroll's body was taken to a police garage in Paisley.
He told Mr McSporran, prosecuting, that Mr Carroll's body was searched for firearms, but none was found.
Mr Withers added: "As his clothing was being removed two bullets fell out. They had entered his body, exited and then fell from the clothing."
The jury was told that Mr Carroll was shot through the forehead, the jaw, the chest and the groin during the 25-second killing.
It is alleged that while masked and acting with others, Mr Monaghan murdered Mr Carroll by repeatedly discharging loaded handguns at him, shooting him on the head and body.
He is further accused of, while acting with others, attempting to defeat the ends of justice by disposing of a revolver, pistol and ammunition within undergrowth in Coatbridge and Airdrie, both Lanarkshire.
It is also claimed a car - bearing false number plates YF 55 EZZ - was set on fire.
Mr Monaghan also faces a number of firearms charges.
He denies all the charges against him and has lodged a special defence of incrimination against eight men.
The trial before judge Lord Brailsford continues.