Church organist Alan Greaves was struck with such force the plastic inside his cap melted because of the friction, a court heard.
Forensic scientist Samantha Ashanti-Warner said damage to the cap showed Mr Greaves had been struck with a weapon such as a claw hammer or a crowbar with a double-clawed head.
Mr Greaves, a 68-year-old married dad-of-four, was assaulted in High Green on Christmas Eve as he made his way to a midnight communion service at St Saviour’s Church.
Ashley Foster, aged 22, of Wesley Road, High Green, is on trial accused of his murder. He denies the charge.
Jonathan Bowling, also 22, of Carwood Way, Pitsmoor, has admitted the killing.
Miss Ashanti-Warner said Mr Greaves’ hat had two cuts which had penetrated the fleece to the plastic inside.
She said: “In my opinion these areas of damage were caused by an implement with a sharp cutting edge. Furthermore, the force of the blow generated sufficient heat to melt the plastic.”
Asked by prosecution counsel Robert Smith QC if the cuts could have been caused by either of two pick axe handles recovered by police, she said: “No, not in their current state. They have curved surfaces.
“In my view the damage was caused by an implement with one or more cutting edges.”
She added: “Typically, the type of implement I would have expected to have caused the damage would be something like a clawhammer or a crowbar with a double-clawed head.”
David Cooper, from High Green Methodist Church, gave evidence via videolink from Rhodes, Greece, where he is on holiday.
He said on Boxing Day he found two pick axes had been stolen from the church shed.
The trial continues.