I've been shooting once or twice a week for the past 18 months, and reloading just as long, which by no means makes me anything more than a novice. Having made my disclaimer, I am happy to add to the conversation nevertheless.
I shoot the Vmax and Zmax in 223 and I find the Zmax varies in wt by several tenths of a gram wereas the Vmax is very consistent. If you not sorting your Zmax by weight, that might explain what your seeing. Otherwise, your "test" was shot on different trips to the range and that by itself could explain the difference.
IMO Varget is a bit slooooooow for 50 gr. Your bullet is probably getting a good sized kick in the butt as it leaves the barrel due to unburnt powder (exit blast). Hard to argue about that group though, as long as it's consistent, that's worth crowing about (see what I did there…LOL)
I'd shoot 10 each 5 round groups before being satisfied it's a keeper load.
I use 3031, 8208XBR or Tac for my 50 to 55 grain loads. N-133 is also a good one to try if you can find it.
Save the Varget for heavier loads. IIRC it was made for the 308.
Almost forgot to answer your question about seating depth. I tried varying seating depth in 40 thou increments. I saw lowering POI as I got deeper into the case until I hit 120 thou deeper, then group hit much higher. Must have crossed that magic threshold where pressure increased, probably due to changed packing density of propellant.
Indeed, it is rocket science.
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