What effect will seating depth have on my velocity. I have seated out to maximum magazine length and am getting 3450 fps. What will happen to velocity if the only thing I do is to seat the bullet deeper, all else staying the same?

In most cases, this will result in an increased velocity. Seating the bullet deeper into the case reduces the case available volume, and the charge will burn a bit faster and reach a higher pressure. In fact, it can really bring up the pressure fast if you are near or at maximum load levels already, so be careful. On my .223 AI, seating 0.100" farther out (I had the throaght lengthened for heavier bullets) required a charge increase from 24.5 to 25.5 gr of the same powder to get the same velocity.

At one time I thought that there was an effect of OAL, but it turns out that if the bullet gets stuck and movement is delayed, that delay is a bigger effect than the combustion volume.
What does it all mean?
When you test with different seating depths, you must be sure that is all that is changing.
Bullets can also bulge from seating force and be too fat for the chamber.
Bullets can also get stuck in the lands.

So if the seating depth changes velocity, it is because it changed peak pressure, but maybe not just because the combustion volume changed.

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