That's something worth noting too.
I seem to remember having mentioned this not so long ago. That after so many shots,the rifle would have to be cleared of " soot " ( debris ),then oiled.Due to this procedure not having been done,there'd have been no oil to protect the workings of the inside of the rifle,leaving it in a " dried-out " state thus not producing the said residue that was missing from Sheilas' nightdress.
The manner in which the rifle was fired WOULD leave it hot ( overheated ) enough to burn flesh,and would have shown a discolouration on the metal too,not to mention the damage to the overall efficiency when firing.
My theory is that Sheila could possibly have attempted to clean the inside using the tampax ( which she was using herself at the time ) I honestly couldn't imagine Jeremy cleaning it out that way on account of fibres getting into the workings ! There would have been a proper " tool " for pulling through,of which there was no sign of.