‘A firearms expert would be able to explain it better, but basically every gun barrel is rifled during manufacture. The rifling process creates spiral grooves that run along the barrel and improve a bullet’s accuracy as it rotates during flight. A fired bullet goes out through the barrel and ends up with mirrored markings on it, which match the rifling on the inside of the barrel . . . Am I making sense?’

‘Yes, I think so. The principle sounds the same as striation marks left on bones when a body is cut up,’ she said, remembering a case she had had where a body was dismembered with a hacksaw by a dentist.

‘That’s right, if somewhat gruesome. There are several methods used in rifling a barrel, which in turn makes a revolver or semi-automatic unique in its own way . . .’

‘Like a sort of fingerprint?’ she asked.

He nodded, and she continued.

‘So, if we recover the gun that fired the bullet, the barrel can be examined to see if the rifling marks on the bullet are the same.’

‘Sort of, but not quite like you described. The lab will test fire the suspect gun in water, then compare the test bullet against the ones recovered from the scene of the shooting. If the markings on the test bullet match the suspect ones, then you know you have the gun that was used in the robbery. But even if we don’t recover a gun, the marks on this bullet can help identify the type and model of firearm that was used.’

‘How do they test fire in water?’ she asked, imagining someone in a swimming pool firing a gun at a target.

‘You’ll see when you come with me to the lab, so I won’t spoil it by explaining.’

He reached into his jacket pocket and brought out a small round plastic container in which he placed the damaged bullet. Handing it to Jane, she put it in an exhibits bag, then signed and marked the bag JT/8. She also entered the details in the exhibits book.

‘What about the skid marks – will they be of any use for tyre impressions?’ she asked.

‘I’ve already photographed them, though they won’t be a lot of use unless we find the car the robbers were in – even then it was probably a nicked or ringed motor. Ideally, we’d need evidence like their fingerprints, or fibres from their clothing inside the vehicle, to physically put them in it . . .’

‘A young girl I spoke to, who witnessed the incident, said the two men she saw get out of the Cortina wore masks and gloves; they were also dressed the same in donkey jackets and blue overalls.’