Interesting video. I wonder what difference there is when the rifle is not locked to the sled. It appeared as though by restraining the butt of the rifle, the effect was emphasized. The side ports probably did a better job on the tested bolt action rifles. The top ports may have been better used on a shouldered semi automatic rifle with a subsequent follow up shot. Your tendency would be to pivot upward at your shoulder, which would be reduced by the top port.
I think being in the sled, magnifies the effect. a person holding said rifle, might absorb enough of the shock as to make filming / detection of the deflection almost impossible. Maybe..... I R No expert.
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