I'll bet the equipment used by the likes of Winchester cost in the multi-thousands range, and for a good reason(s).
Is Sherman's equipment adequately responsive to measure true peak pressures? A lot of the difference between expensive test equipment and cheaper stuff is the ability to consider rise time. In other words, the pressure may peak at a much higher level than he is reading, and drop off, before his device knows it happened. That could explain the difference between his readings and those made by the big guys.
As noted by GJZ, the equipment used is the same as the 'big guys'. Armbrust uses an industry standard pressure gun. Armbrust is the ballistician used by the 16ga group to test all of their loads. Pressure gathered by Armbrust and reported by Bell can be taken to the bank.
