This produces bogus readings as you're leaking two whole barrels worth of gas which leads to a worse seal and lower reading of the one shot that does happen, but if the site's happy with the results then it's good lmao
TM/GE 870s + the KSG have restricted output built into them anyway, the barrels don't run the entire length of the barrel + there's some sort of thing with the valve inside that prevents the power going above 280 or so FPS on standard gas so I wouldn't even worry about the reading being lower than stated anyway as it's way below limits