I've noticed that too in my 12FV, 223. Seating depth changes that large tend to change barrel time, max pressure, and velocity. It'd be interesting to chronograph those loads and see what kind of velocity change you're experiencing.
I performed a seating check using the .040 technique where you seat one group of loads touching or lightly into the lands, next one .040 off, next is .080 off, etc. and as I recall I went out to .140 off.
I started by shooting the group loaded to the lands, and worked my way off. All groups printed lower on the chart than the previous, with the exception of the last ones loaded to .140 off. Those printed much higher and tighter, but also cratered the snot out of the primer. I did not own a chronograph at the time, so I had no data to explain the higher POI.
Using this method, one of the groups will shoot much better than the rest, and that is the distance you begin to refine for that particular bullet.
Here is some interesting reading;
http://www.the-long-family.com/optim...rel%20time.htm
http://bulletin.accurateshooter.com/...e-time-factor/
And if you're really having trouble falling asleep some night: http://archive.org/stream/velocitypr...0culv_djvu.txt
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