Khuram Shaikh, a Red Cross worker from Rochdale, was found with gunshot and stab wounds on Christmas Day in 2011.
The expert, Mr Gunatilleke, told the trial in Colombo the residue was found when he wiped a wound on his forehead.
Sampath Vidanapathirana, a town council leader, and five other men deny murder.
They also deny raping a woman.
Mr Gunatilleke, a witness for the prosecution, said five spent bullets were found at the scene of the crime.
He told the High Court there had also been attempts to falsify the serial numbers on the guns and pistol in the defendants’ possession, but he had used chemical methods to discover their original numbers.
During last week’s testimony, photos taken by Tangalle Police depicting the wounds to Mr Shaikh were shown in court.
The case resumes on Wednesday.