The original poster wrote "When Jack Harmer was "shot" forensics don't seem to have checked the shirt for substances, which would have given the whole plot away."
But he was in fact shot through the heart, and this was only a matter of seconds after the 'fake blood' simulated shooting. The quantity of real blood would have massively overwhelmed and flooded out the fake, which was supposedly some sort of red sugar syrup. It came from a tiny sachet, so the volume would have been minuscule. Not really a goof.