The M3 is doing strange things, also I noticed groups opening up the closer it gets to the 2nd reg pressure, Mk2 impacts did not have this issue.
I was checking a zero at 40 yds the other night and it was slug on slug, then a shot threw 4" high on two occasions, all be it I was shooting faster than normal, some say this can affect FPS, but I wouldn't expect two shots 4" high on occasion.. Also I've watchd the 1st reg stick at random pressures on the gauge a few times too...