This question comes up again and again. The answer is quite obvious.
Either Jeremy did not notice the minute amounts of blood on the moderator or assumed such minute amounts of blood would not matter because police would not search for the moderator and would not think it had been attached because it was not on the gun when they entered and make up his story about leaving the gun out without the moderator attached.
It is quite easy to miss the minute amounts of blood that were on it and also easy to see why one would expect such to not hold any significance. Even after police were made aware human blood was on it they didn't think much of it.
He knew nothing about drawback and thus had no idea that blood was inside let alone that blood inside would be able to prove the moderator was attached during Sheila's fatal shot thus proving she was murdered. If he had known about drawback then he would have removed the moderator before killing Sheila so that blood would have been found in the rifle itself. The only reason he removed the moderator at all was because after placing it on her body he realized it was too long for her to shoot herself with it so that was why he removed it and hid it.
Some people want to make up that he knew about drawback and thus would have washed the moderator to but such people are simply making such up because they don't want to face the moderator was used and prefer ridiculous fantasies instead of choosing to face reality and look at things realistically through Jeremy's eyes.
These same people want to pretend the police knew all about drawback though they clearly didn't. When the lab explained drawback, explained that Sheila's fatal wound would have resulted in drawback, and that Sheila's blood not being found in the rifle but rather in the moderator proves it was attached when the fatal blow was delivered this surprised police. Only the senior investigators had it explained to them, the others heard it through the grapevine and didn't even fully comprehend it.
The family wasn't told the significance and only learned when they listened to the trial testimony.
That people say Jeremy should have known about drawback and thus should have washed it out is absurd he had no way to know and if he had known about drawback and that it would result from the fatal shot then he would have delivered that shot sand moderator because it would not be enough to simply clean out the moderator he would want her blood in the rifle itself. Where drawback would occur and yet none is found in the rifle it is a big problem either a moderator was used or many bullets were fired after. It would take a large number of shots for blood to be completely removed from the barrel by virtue of subsequent shots fired. Even after 4 shots blood still had been detected in testing and real world conditions. If more shots were fired after her death it means she can't have been the one doing the shooting. So anyone in Jeremy's place who actually knew about drawback would have made sure to fire the fatal shot without the moderator attached. But drawback is obscure and even today not widely known about let alone would Jeremy have any way to know back then.
