The scientific evidence in question is gunshot residue
The Rhys Jones murder trial has been told questions over the gun allegedly used to kill the schoolboy delivered a "crippling blow" to the prosecution.
Richard Pratt QC, defending Sean Mercer, said a scientist who examined residue from the firearm had accepted it may not have been the murder weapon.
Mr Pratt told Liverpool Crown Court that the question "struck at the heart" of the case against his client.
Mr Mercer, 18, of Good Shepherd Close, Croxteth, Liverpool, denies murder.
Rhys, 11, was shot dead as he walked through the car park of the Fir Tree Public House in Croxteth Park on 22 August 2007.
Delivering his closing speech, Mr Pratt warned jurors to suppress feelings of outrage or sympathy in deciding whether his client was guilty.
He also told them not to read anything into the fact that Mr Mercer had not given evidence.
On behalf of Sean Mercer, we will suggest to you that on the totality of the evidence you should not convict him
Richard Pratt QC, defending
He said it was inevitable jurors would feel anger at the killing, but that they must remain dispassionate.
Mr Pratt told the seven women and five men: "Don't be driven from your task from feelings of sympathy and outrage."
Mr Mercer, along with four of the six other defendants, refused to give evidence from the witness box and face cross-examination.
Mr Pratt said: "He, like others, has exercised his right not to give evidence in this case.
"That, as I said at the time and emphasise now, is a right. Exercising that right is of course not without its risk factor.
'Not guilty'
"I cannot even tell you of the reasons which caused him to exercise his right not to give evidence."
Mr Pratt said prosecutors labelled evidence given by Melvin Coy and Gary Kays, both aged 25 and who deny assisting an offender, as "pathetic".
"So from the prosecution's point of view, you are damned if you do and damned if you don't," he said.
"We hope for a more balanced view from the jury," Mr Pratt said during his closing speech after nearly two months of evidence.
He also reminded the jury that his client had not remained totally silent, giving police a prepared statement through his lawyer.
Mr Pratt said Mr Mercer said "two all important words - not guilty".
"On behalf of Sean Mercer, we will suggest to you that on the totality of the evidence you should not convict him."
Those on trial accused of assisting an offender are James Yates, 20, of Dodman Road and Nathan Quinn, 18, of Wickett Close, both Croxteth; Mr Kays, of Mallard Close and Mr Coy, of Abbeyfield Drive, both West Derby, Liverpool, along with Boy M, 16, and Boy K, 17, who cannot be named.
Boy K is also charged with possession of a firearm and ammunition, which he also denies.
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