I suspect the only way this could be improved is to remove variables, i.e. taking the action and barrel out of the human handle and bolting it to a big chunk of steel on a slide setup with a positive return to battery to reset after each shot and some form of mechanical device to operate the trigger to take the finger out of the equation.
That's going more to the proof barrel idea though, and I have heard of random examples where people have bolted rifles into sleds and got one hole groups and expected the same when the setup is hand held and been hugely disappointed that no human can shoot the thing near as accurately. Put down to the strapping into the sled doing something to the bedding or other variables that human hands and shoulders can't... It's getting outside of the original testing hypothesis though.
As far as does shooter fatigue contribute to larger groups - yes. It's been proven several times over and there are several sources of fatigue that do it - eye fatigue from not blinking enough or focusing too long or optics that aren't set quite right for your eye, muscle fatigue, cumulative effects of recoil and blast, excitement and/or pressure on weird bits of the body from lying over something awkwardly all play a part as does nutrition and hydration. I can't shoot for nothing when I need to take a piss for one!
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