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Claims were made on TV3's 3rd Degree programme in June that the marks shown in photographs were made by loading the rifle's magazine.
But police said scientific analysis by fingerprint and firearms experts did not support that conclusion.
Police have released an analysis of fingerprints taken from Robin Bain plus a report on testing of the gun used in the Bain homicides.
Fingerprint experts examined the original post mortem fingerprint forms taken at the mortuary and compared those to the marks in photographs of Robin Bain's thumb and forefinger.
That examination confirmed features in the fingerprints corresponded accurately with the marks shown in the photographs, police said today.
The experts concluded that this was strongly indicative that the marks shown in the photographs were the result of minor superficial damage to the skin surface.
Tests were also carried out by an ESR firearms expert on the weapon used in the Bain homicides, aiming to replicate the marks shown in the photographs.
The scientist's opinion was although there appeared to be a pair of lines on Mr Bain's thumb that could have resulted from loading a magazine cartridge, there was considerable doubt that the shape, dimensions and colour of the marks on Mr Bain's thumb were consistent with marks made as a result of loading a cartridge into a magazine, police said.
"These tests have been carried out using accredited experts in controlled conditions with access to the original exhibits and in the presence of Mr Bain's advocates,'' assistant commissioner Malcolm Burgess said.
"The most likely explanation for the marks on the photographs would seem to be pre-existing damage or injury to the skin on Robin Bain's thumb.
"I am satisfied that this scientific analysis shows the marks highlighted by the 3rd Degree programme are anything but the 'game changer evidence' the programme claimed,'' Mr Burgess said.