When speaking about it he said they appeared to be burns.
What is a burn type mark? It is a mark that can be caused by a burn but can also potentially be caused by something else. Injuries like marks caused by burns are extremely similar to other wounds. Moreover there are different causes of burns- thermal, chemical, radiation, electricity, friction and so forth.
How do doctors diagnose a burn? The number one method of diagnosis is they ask the victim how the wound occured. What about when the victim is unable to speak or dead? The circumstances of their death can give it away such as when there was a fire or chemical explosion.
When a doctor has none of this the only way to diagnose a burn is to actually test the wound. Test the flesh for singeing and see how far the wound extends and test for the presence of chemicals that can burn. Third degree burns are of a nature that it is easy to say is a burn because no other wounds do that extensive damage. 1st and second degree burns are topical and the mark left can be similar to other wounds so visible identification may not necessarily be enough.
But given his level of expertise we can assume they probably are burns. We can't say for sure since no in depth analysis was done. The photos are consistent with 2nd degree burns.
Here is a second degree thermal burn, notice how it looks like a bruise. The dark inside skin can look bruised from a distance you need a good inspection to tell the difference between bruising and burning. With friction burns this is even more so. The outside area is pink and starting to get infected. This is a relatively fresh wound not a scar. A scar looks totally different which is why the report doesn't call the marks scars they are not scars.
Though they are probably burns they might not be and if burns there are a host of different causes possible. They might have been suffered shortly before the murders (definitely not a long time before as you claim) but more than likely were inflicted during the murders.
It takes 17 seconds of contact at 55 degrees Celcius (131F) to cause a second degree burn like this. It takes 3 seconds of contact at 60 degrees Celcius (140F) to cause a second degree burn like this.
Would the murder weapon get that hot after 10 shots? No. Would it get that hot after 20? If the right ammo is used it might but not likely with the ammo in question.
At any rate the hot object that made this mark is not hollow in the center the heat was applied to the center and went out to the edges. A rifle barrel is hollow in the center which is why the test has a middle of the burns on the pig doesn't have singeing.
Interestingly electrical burns look very similar. They often are spherical, here is a taser burn:
Did they have a cattle prod or stun gun or something of the like? There are just too many variables to determine how these marks were made let alone for them to establish anything significant as far as the commission of the murders is concerned. It is thus a red herring at best. The location on the body is also so far up that if made by the killer it would have likely been done while Nevill was already down. Either to keep him down or to check if he was still conscious. Even Jeremy would have held the rifle or a prod lower if jabbing him to get him to move.
A taser or the like though could have been shot into him while still standing.
