He pricked his own finger and used that

What is remotely amusing about that? You do not appear to understand the blood evidence and what was available to police investigations in a pre dna era. The only tests available were based on serology which is incapable of producing anything statistically individualsing. The nearest being groupings which are shared by 8% of unrelated white British population. Even then it requires a large sample of the right quality. Hence when the blood stains were analysed on the rifle they were incapable of being typed:
71. The rifle bore blood smearing on the barrel in the region of the fore-sight and around the mechanism and there were splashes of blood to the left side of the weapon. The appearance of the blood staining was consistent with it having been used to strike somebody who was already bleeding. On analysis the blood was found to be human blood but tests to determine grouping were unsuccessful.How many times do you think officers needed to send items to the lab only for them to come back with 'tests to determine grouping were unsuccessful' to fully understand what was required to get a result?
DS Jones would have been only to aware that tiny traces of blood would only show up as blood and human in origin. How would his blood implicate him?
Note the size of the stains on the rifle v the outside of the silencer,