MacDonnell knew about the moderator, but through a misunderstanding in communication thought that it was attached to another rifle. His other general remarks about the bible, blood flow and trails still stand as sound reasoning that Sheila didn't kill herself.
I suggest we apply for a firearm's licence, all chip in to buy an '85 vintage Anschutz 525 + Parker-Hale moderator, and experiment on something living... so you'll be fully satisfied that drawback (and backspatter) does occur at close range.
... or maybe not, because then you'll get a bee in your apostolnik about something else.
http://watchingyouwatchingyme-steelmagnolia.blogspot.co.uk/2011/02/bamber-report-from-prof-herbert-leon.html
The fact there was a misunderstanding in communication and MacDonell made his assessments based on the wrong firearm does not fill me with confidence. In effect the whole thing can be written off. With regard to the aspects he reviewed that were not connected to the firearm he hasn't taken into account that it's pretty much a racing cert EP moved SC, the rifle and bible before photos were taken. It was some 2.5 years ago when I read Wilkes book but I think I recall MacDonell's report was some sort of budget version and the real McCoy was going to involve some significant outlay. To my mind it seems strange that JB would commission a world renowned expert such as MacDonell knowing he was guilty?
I have for the first time Googled various permutations of 'draw-back' and 'drawback' along with other relevant words eg firearms, gunshot, etc and there's very little info. This seems to support my claim that the phenomenon of blood being sucked back into a firearm with or without a silencer is rare.
Blood pattern analysis seems an important consideration and yet in the CoA document we are told there was a considerable amount of blood and a flake:
76. Inside the moderator, on the four or five baffles nearest to the end from which the bullet would exit, there was a considerable amount of blood. At one point blood had pooled to form a flake when it dried, and this flake was subjected to group testing. Results were obtained for four of the five tests performed. Mr Hayward, the forensic scientist said that they showed that the blood could have come from Sheila Caffell but not from any of the other individuals involved. Mr. Hayward said that there was a possibility that the blood could be a mixture of blood from more than one person and if it was, a mixture of blood from Nevill Bamber and June Bamber could account for the findings in the grouping tests. However he judged that possibility to be a "remote" one.
It sounds a bit basic?
MacDonell has stated that draw-back with a .22 could travel as far back as 2.53 cm to 3.8cm. The length of the Parker Hale silencer is 17.3cm. The blood was found as far back as the 5th baffle so does this take it beyond the 3.8 cm? How far back is the 5th baffle? It appears there are 15 baffles with a Parker Hale so assuming they are evenly spaced and allowing a bit extra either end it sounds to me although 5 baffles in would take it beyond 3.8cm?

"Results obtained from the research of MacDonnell and Brooks revealed that
penetration depth of small calibre weapons like .22 calibre revolvers, ranged from 1 to
1,5 inches (2,53 to 3,8 cm). The depth of detectable blood droplets in higher calibre guns
such as 12-, 16-, or 20-gauge shotguns reached up to 5 inches (12,65 cm) and for
handguns approximately 3 inches (7,6 cm)"
Also as the draw-back phenomenon is based on suction not only is blood drawn back into the silencer but also other biological material but none were present:
"The drawback effect (or blowback effect) is a phenomenon seen mainly in contact
and close-range gunshot wounds. It refers to the process in which blood droplets are
deposited inside the barrel of a weapon after discharge. The cause of this phenomenon
has not yet been agreed to, but current theories include the increased pressure in the
gunshot wound and the near vacuum in the barrel (created by the discharged gases
escaping along with the projectile), which then creates a sucking effect that sucks blood
and other biological tissue into the barrel".