That spread in velocities tells me either: Ignition problems, or Chronograph problems.
Run some commercial/military ball over the chrono and see what you get.
Even at 16" you should approach 2,900 w/ SD's ~20-ish.
If still reading low, you've isolated the problem. Check battery (my Oehler like lithiums when cold)
If reading comes back "normal," up the powder charge and see if ignition improves. (You're already running a mag primer)
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