I don't doubt that. But 'drawback aka as blowback' is a complex phenomenon where the opposite of what would ordinarily be expected occurs ie gases causing blood to travel towards the line of fire. The idea Mrs Eaton woud have such knowledge is laughable! 
What has this got to do with the above? You're conflating two entirely different propostions: one how you believe the silencer was contaminated and two that the silencer wasn't used.
In any event the expert evidence from the defence in this regard hasn't been challenged by the prosecution.
Hair can impede flight and you also need to factor in an immature skull.
This is one of the central problems of the theory that the silencer was planted by the family. It has been discussed before, and when I can, I may pull out my own posts on the issue. Here I will summarise a possible response that the pro-Jeremy camp can make to this, and another central problem.
I am non-committal on the matter. I think there is evidence that the blood distribution within the silencer is consistent with the blood having been planted, but that does not mean the evidence was planted. Furthermore, I think if this evidence was planted, it should still be possible to prove it at some level, but at the moment, I regard the position as uncertain.
I am not sure that I would accept the idea of Anne Eaton as a conspirator. If there was a conspiracy, I would assume it would involve Robert Boutflour, Jnr. and David Boutflour only - father and son, sworn to secrecy. But I am not saying they did this, or anything. I don't know! I don't want myself or this Forum to be sued for libel.
The argument from guilters is that the family could not have planted the silencer because:
(i). they were not cognisant of the drawback phenomenon; and,
(ii). they would not have appreciated the evidential significance of the silencer itself;
(iii). they did not have access to Sheila's blood.
Let me add another strong pro-guilt point that I thought of when I was going over this in my head:
(iv). How does the family know what the police have or have not checked, seen and observed when looking in the gun cupboard and examining the various firearms and paraphernalia? For all the family know, police officers may well have already examined the silencer in situ and decided it should bear no significance to the investigation. The police may also have indexed and recorded their findings.
Even if you are pro-innocent and want to dismiss these points, let's agree on something: if the silencer was planted, whoever was responsible was taking a calculated risk. But as David observes, criminals do take risks.
Now, here is a possible response that the pro-Jeremy camp could make to all this:
In regard to (i), the family were not experts in forensic ballistics, but they didn't need to be. They were gun users and would have enough knowledge and intuitive understanding to appreciate the significance of the silencer. They would also intuitively understand that any sort of blood found in the silencer - especially human blood of any type - would throw the investigation a wobbly and potentially cast suspicion on Jeremy. Remember that the aim may have been not to incriminate Jeremy at all, but merely to cause issues and trouble for him, cast general doubt over his innocence (if not legal doubt, then at least a shade of factual doubt), and damage his credibility during a period when he was vulnerable. That kind of evidence may also be useful in civil proceedings against Jeremy over the Bamber-Speakman estates, in the event that Jeremy is not charged, or he is acquitted following a criminal trial.
Turning to (ii), there are two threads to the objection. First, how did they know that the rifle could not have been used by Sheila to kill herself with the silencer attached to it? Second, how did they know to put Sheila's blood in the silencer instead of, say, Nevill's or June's? The reply to the first point is that they did not even need to think about it. Their issue was that the silencer was missing and they then found it. It was Stan Jones who came up with the idea that the rifle with a silencer attached was too long for Sheila. The reply to the second point is that they didn't need to be concerned with whose blood it was, because:
- any sort of blood found in the silencer would cast suspicion on Jeremy;
- human blood of any type found in the silencer would probably lead to Jeremy being charged, but not necessarily convicted;
- human blood of the right type found in the silencer could be enough for a jury to convict him, but it depends on the other evidence, as it's mainly a circumstantial case.
Turning to (iii), the problem with this objection is that it assumes the identification of the blood matters. You forget that, given science and technology of the time, the forensic case against Jeremy was probabilistic because the trial could not pinpoint that it was Sheila's blood. Instead, what the prosecution were saying to the jury is:
(a). the blood is human blood;
(b). the human blood is of a type that is the same as Sheila's;
(c). the rifle was legally Nevill's, but in fact it was Jeremy's gun (there is no suggestion Jeremy was using the gun illegally during the normal course of things, as he could assert a statutory dispensation);
(d). Jeremy admits that he was the last person to handle the gun prior to the incident;
(e). Jeremy admits that he was aware of the incident involving Sheila via a phone call from Nevill, the veracity for which the court is entirely reliant on Jeremy himself;
(f). given facts (a), (b), (c), (d), (e) and (f), the legal onus shifts to Jeremy to explain why there is blood in the silencer, and you, as the jury, must consider any explanation he offer, or no explanation, against all the surrounding facts and circumstances, including the evidence of Julie Mugford;
(g). Jeremy's explanation is that Sheila is the killer and she must have replaced the silencer in the gun cupboard before shooting herself;
(h). in all the circumstances, and weighing up the probabilities, Jeremy's explanation seems improbable;
(i). the criminal burden of proof is that you must be sure, but the improbability of Jeremy's explanation is such that his innocence is at the very margins of plausibility, hence the Crown submits that the jury can be sure of Jeremy's guilt.
You may agree with all this, but the key point is that the prosecution could not be sure it was Sheila's blood on the basis of a specific test for that purpose. It is possible that in fact it was someone else's blood, of the same type or even a different type. I think it follows that the family must have been aware of two things:
- blood could not be identified, it could only be typed;
- even human blood of an entirely different type would circumstantially throw doubt on Jeremy's innocent act.
There are two possibilities:
Possibility A: the conspirators may have just put any old blood in the silencer, on the basis that any human blood causes problems for Jeremy but could not be identified as blood of the family. The coincidence of blood types benefited the prosecution case without casting suspicion on the family, but this was not planned.
Possibility B: the conspirators knew Sheila's blood type from family information and knew Ribetr Boutflour Jnr.'s blood type and also knew from general knowledge that blood could not be identified, it could only be typed, so they planted the 'right' blood in the silencer, anticipating that Jeremy could then be charged with the murders.
One further point to consider is that the family were blood tested by the police prior to the trial. Nobody seems to have suggested that because Robert Boutflour, Jnr.'s blood type matched Sheila's, and therefore matched the weapon, that this means there was an attempt to frame Jeremy.
As for (iv), the family may have been told by the police in casual conversation that nothing had been found. Or they may have just taken the chance, in the belief that if caught out, they could allege that the police had missed that particular silencer. The family had firearms silencers of their own that resembled that one and could easily have added one of these to make it look like there were multiple silencers. Indeed, this may well be what actually happened, given that we know the FSS examined at least two different silencers.
Please understand that I am not saying any of this is the case. I only offer you possible replies to the objections raised.
I have my own private suspicions about it all, but it is for you to decide what you think.