I accept there may have been blood on the stock but what does this prove or disprove?
1) There was spatter that "splashed" the left side of the stock- this proves Nevill was beaten with the stock because as he got hit with the stock the blood splashed onto it. There was very limited spatter on the mechanism and barrel but it was still there. The spatter gets on the person wielding the bludgeoning instrument not merely the weapon. It didn't reach the barrel and yet not hit the killer. This means if Sheila had been the killer she should have had spatter on her clothing and body particularly the back of the hand that was holding the stock.
2) It tells us the killer was wearing gloves or the killer's hands would have had blood spatter on the outside and on the inside of the weapon there would have been prints of some sort even if partials only left in the blood.
3) There was blood transfers to the area where the piece broke off as well as the stock area below the trigger. This means the hands of the killer had blood on it and transferred it to the gun. This confirms the observation the killer was wearing gloves since the transfers didn't involve any prints whatsoever. The killer's gloves had blood on them that transferred to the weapon. Was the blood from spatter? From punching the victim in the face as he was bleeding or something else such as touching the victim to re-position him? Who knows all that matters is that the killer had blood from the victim on the gloves.
Were there any bloody gloves that Sheila could have worn at the scene? No
Did Sheila have any medium velocity spatter on her clothing (clothing she wore at death or any of her clothing she could have changed out of) that could have been from Nevill? No
This is why it is significant. If Sheila had been the one who beat Nevill then there would have been gloves and clothing she wore with his spatter on them. It is further corroboration that the killer fled the scene because no such items were present.
It is part and parcel of the whole issue of what evidence would have been on Sheila's body and clothing if she had killed the others and herself.
Why did the jury reject the defense claims that maybe Sheila changed? Apart from the fact that it makes no sense for her to wash up and change her clothing she changed out of would still be there.
When you add to that the fact that she can't have cleaned off evidence that would be there if she shot herself and the evidence proving it impossible for her to have shot herself it is a slam dunk even before you add Julie's evidence.