I shoot a 338 ultramag with 300g Sierra Matchkings. My chrono says 2770 f.p.s. consistently. I follow my drop chart and it hits low about 5" at 500 yards, 6" low at 600, 7" low at 700 etc. It seems to shoot as if the velocity were more at 2650 f.p.s. or so. Could the chronograph be faulty? I am using a "shooting Chrony" brand chronograph.