[Unclenick]

It's an interesting experiment but curiously uninformed on a couple of points. For example, they seem surprised that velocity doesn't change proportionally with peak pressure. Peak pressure is never proportional to velocity because it's determined by how much gas is made in what degree of expanded space there is behind the moving bullet. As peak pressure goes up, burning rate increases so the bullet has less time to move forward. That means progressively higher peaks occur in ever decreasing volume which makes the pressure growth exponential.

Velocity, on the other hand, is determined by average pressure in a bore, not peak pressure. The average pressure allows the gas to fill the whole barrel by the time the bullet exits, which is a more constant volume than the changing expanded volume extant at the peak. The two are not independent of one another, but it's not uncommon for peak pressure to change three times as much as the average pressure, percentage-wise.

For another thing, they are varying pressure and velocity with a fixed powder charge. This changes barrel time. The higher the peak, the larger the portion of bullet acceleration that occurs early in its travel, so it goes a little faster down the rest of the tube, shortening barrel time. This moves the bullet in and out of sweet spots in the barrel deflection and vibration. It is therefore no surprise to find they can tune to more than one sweet spot by varying seating depths with with a fixed charge.

I thought it was pretty widely known that you can often find a seating depth sweet spot near the lands and another with the bullet bearing surface about one caliber into the case mouth. Obviously some chamber and bullet combinations don't really allow a choice between the two, but it's not uncommonly observed in those that do, much to the relief of those of us who have magazine fed match rifles.