Detectives and firearms experts believe the killer may have taken apart the round used in the murder and removed some of the gunpowder so that it would be quieter when fired in the street.
The police yesterday released details of six small handmade indentations found on the brass cartridge casing that was left at the scene of the murder outside Ms Dando's front door in south-west London almost 100 days ago.
Officers on the case have checked with forensic experts and murder files in Britain and Europe, but there are no records of similar markings having been used before.
The tiny marks which were probably made with hammer blows would have had the effect of holding the bullet in place by "crimping" the cartridge case, but the firing of the lead bullet would not have been affected.
Detective Chief Inspector Hamish Campbell, who is heading the murder hunt, said the markings could have been a "trademark, habit or idiosyncrasy".
He added: "Examination of the cartridge case has shown little individual markings on it which are unique.
"It has six tiny indentations, fairly regularly spaced around the top edge of the cartridge. It has the effect of holding the bullet in place.
"My appeal is to gun dealers, firearms dealers, those handling ammunition. Does the bullet with these markings mean anything to you?
"Has anyone passed on any bullets that have this unique marking. It's been tampered with, but it isn't hand-made."
Det Ch Insp Campbell refused to speculate on the possible motive of the killer or disclose what the police's forensic experts had concluded about the markings.
But a firearms expert suggested last night that the marks left on the cartridge may have been the work of a professional killer who customised the bullet by removing gunpowder to make it quieter.
Brian Carter, director of the Gun Trade Association, said: "Reports have suggested that no one heard a shot - one explanation could be that a professional hitman took the round apart and took some of the powder out so that there was enough to kill somebody but not enough to make a big bang."
He added that the "crimping" pattern found on the top of the casing could have been created by the gunman to ensure the bullet stayed tight inside the cartridge.
If the round was doctored to reduce the sound, it suggests that the killer was a professional criminal with knowledge of firearms and specialist tools. An alternative explanation could be that the gunman was an obsessive fan of Ms Dando who has left a form of signature on the round as a sick joke.
Police know that the gun used in the killing was a 9mm "short" semi-automatic handgun, makes of which include the Beretta and the Walther PPK.
The bullet and cartridge used was a Remington-make 9mm "short", which is relatively rare compared to the standard 9mm, the most commonly used handgun round in the world. The bullet used to kill Ms Dando had not been altered in any way.
A police source said the murder inquiry team still believed Ms Dando, 37, was shot in the head by one of the following: a stalker, an obsessive fan, a mentally ill person or a contract killer.
Ms Dando was murdered more than three months ago on the doorstep of her home in Gowan Avenue, Fulham.
The killer was away from the murder scene in about five seconds and there were no witnesses. An e-fit likeness released by police soon after the killing was of a well-dressed man seen sweating at a nearby bus stop minutes after the murder.
If it was a lone stalker, a police source said, only a handful of people might have a link with the person who fired the gun.
More people would have been involved if it was a contract killing: with a gun, ammunition, a getaway car and phones needing to be organised.
It also emerged yesterday that detectives have questioned the Ealing Vicarage rapist, Martin McCall, about Ms Dando's murder. McCall, who was jailed in 1986 for attacking Jill Saward, a vicar's daughter, worked as a security guard at the BBC's Television Centre earlier this year. There is no evidence to link him with Ms Dando's murder.
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