The nightdress must have had residue on it. There are photos of the gun laying on the nightdress.
51. Mrs Caffell's nightdress was bloodstained. When tested the blood was consistent with being her own blood.
The garment was also examined for the presence of any firearm discharge residues or oil from the rifle. No such traces were found. The scientist gave evidence that there would be a strong chance of finding such residues or markings on the clothing of an individual who had fired a rifle twenty-five times.
Why would gun shot residue be on Sheila's nightdress if all Bamber did was put the rifle on her ? Anyway there was no GSR on her.