Rhys Jones was shot outside the Fir Tree pub
A weapons expert has told the Rhys Jones trial there is "a possibility" the gun recovered by police was not the firearm used to kill the 11-year-old.
Angela Shaw told Liverpool Crown Court there was insufficient evidence to be certain which gun killed Rhys.
She said two types of firearms discharge residue (FDR) were left at the murder scene, but only one type was found inside the recovered gun.
Sean Mercer, 18, of Good Shepherd Close in Croxteth, denies murder.
Rhys was shot outside the Fir Tree pub in Croxteth as he returned from football practice on 22 August last year.
Earlier in the trial it emerged the gun had been found in the loft of the home of a 17-year-old witness who has been given immunity from prosecution.
The prosecution claims alleged gunman Mr Mercer fired three shots from a World War I Smith and Wesson revolver across the pub car park.
The first hit a parked BMW, the second struck and killed Rhys, and it is thought the third hit a stone wall.
Ms Shaw was given the role of examining the FDR - tiny particles which are created when a bullet is propelled from a gun.
Giving evidence Ms Shaw said she found one type of FDR - referred to as type one in court - on shattered glass from the BMW and a second type, referred to as type two, on the England football shirt Rhys was wearing when he was killed.
However, swabs of the barrel and cartridge of the Smith and Wesson gun, recovered by police, found only particles on type two from inside the weapon.
Ms Shaw added "The expectation would be that both type one and type two would be found."
Prosecuting, Neil Flewitt, QC, asked if it was possible that the police had recovered the wrong gun.
Ms Shaw replied: "That could be a possibility."
The jury also heard evidence from Andrew Laws, a forensic imagery consultant, who compared CCTV images of the gunman's bike to the frame of Mr Mercer's bike recovered by a member of the public.
Mr Laws told the court there were some differences and was not able to say if the frame was the bicycle in the CCTV footage.
"The images tend to suggest that the bicycle in the CCTV and recovered frame are of the same make and model," added Mr Laws.
Six others - James Yates, 20, of Dodman Road; Nathan Quinn, of Wickett Close, both Croxteth; Gary Kays, 25, of Mallard Close and Melvin Coy, 24, of Abbeyfield Drive, both West Derby, Liverpool - are accused of assisting an offender along with Boy M, 16, and Boy K, 17, who cannot be named.
All deny the charges.
The trial was adjourned until Tuesday.