I don't believe you ACTUALLY showed a loss of velocity, I believe it was a natural over-lap of random numbers due to such a small sample.
I like to shoot a series on the same target, if the rifle is accurate enough not to mix the groups, which yours is.
I make my own targets, but an NBRSA target is good, as it has six aiming points on each piece of paper. If you have a crosshair, aim at a corner of the square. "aim small, miss small." the corner is infinitely small.
If you use a dot, use a circle the same size as the dot.
I use several targets depending on which rifle I am using,ie 4MOA/inches using 2 1/2x scope on my 7x57 Mannlicher-stocked "woodsey" rifle, or 1/2" at 200yards for my long-range varmint 6.5/06 IMP with a fixed-power 20x and a 1/4MOA dot.
This way you can see any changing in the shape of the group as you increase the charge. Later, you can do the same thing if you run a series with changes in the jump.
The barrel tends to form larger, circular groups that change size as you get closer to the sweet-spot, then then they get larger, again. If you can gain enough velocity, you will get to another sweet-spot. The only thing to stop you is when the primers pop, cases split, and cases melt/stick, lol. Results vary depending on the weight/bedding of the barrel
Have fun,
Gene
