Sqrl,
reading thru the list of shot velocities, I see a couple things....
Group one had the first shot at 350, then the sceond shot at 550.... remove both of those and recalculate everything. (remember to reduce shot count by 2 though....). The 350 fps is an outlier, not part of an average string. (ALWAYS... if youi drop low numbers off the list, you also have to drop the same number of shots off the high end)
Scanning the rest of your data groups, Group 2 Id have not incldued the 3rd mag’s data, since its already below the rest of the strings’ velocity.
Looks like group 7 was the most consisitnet, but even then I see a low number that shouldbt be included. But if you drop the the lowest 3, 529, 528, and 520 at the low end, AND drop the 544,544 and 543 off the high end. Recalulate and check the numbers.
IMO, once you see the numbers drop below the other numbers, STOP. Those are not valid data. Even with those six removed from, thats 3 mags of shots at a very consistent speed.