Notice his face is nowhere near close enough to get nitrates on it.
Show us a shouldering position that would have to be taken in order to qualify as being 'close enough' to be sprayed with nitrates.
EISENBERG: A paraffin test was also run of Oswald's cheek and it produced a negative result.
CUNNINGHAM:Yes.
EISENBERG: Do your tests, or do the tests which you ran, or your experience with revolvers and rifles, cast any light on the significance of a negative result being obtained on the right cheek?
CUNNINGHAM: No, sir; I personally wouldn’t expect to find any residues on a person's right cheek after firing a rifle due to the fact that
by the very principles and the manufacture and the action, the cartridge itself is sealed into the chamber by the bolt being closed behind it, and upon firing the case, the cartridge case expands into the chamber filling it up and sealing it off from the gases, so none will come back in your face, and so by its very nature, I would not expect to find residue on the right cheek of a shooter.
