Not contradictory, just not intuitive because its not linear, but it's how the physics of the beast work out. Velocity depends on the average pressure at the bullet base during its whole trip down the tube. You can get to the same average by combining a higher peak with a lower muzzle pressure or vice versa, giving you two different pressure profiles that produce the same muzzle velocity. The first accelerates the bullet more early in its trip, while the second does more of the accelerating later in the trip. The first profile thus has the bullet traversing the second half of the barrel in a shorter time because it gets there already going faster than the first. That's why it produces a shorter barrel time even though the muzzle velocity is the same.
Usually that first and second pressure profile is produced by fast and slow powders, respectively. But when you make a change in seating depth that requires more powder to reach the same velocity, then you are using a lower peak pressure value that makes the powder burn more slowly, and thus acting like a slower powder would, and garnering a longer barrel time for the same reason.
As to the the sweet spot in barrel time, yes, that's muzzle displacement timing. The thinner and more whippy the barrel, the more it matters. You can see Varmint Al's FEA animations to see the whip initiated and to learn the frequencies (roughly third harmonic) and rates of displacement. He's also got some nice animations of all the minor modes.
That said, there are a lot of guns for which simply matching velocity may be close enough. A lot of the barrel time sweet spots are several tens of microseconds wide, corresponding to up to a couple of inches of barrel length difference. That's because the bullets are going so fast that near the muzzle that it takes a lot of barrel length to make a big difference in barrel time). The formula I gave just gets you closer to staying at one barrel time that has already worked out for you.
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Last edited by Unclenick; March 25, 2012 at 04:51 PM.
Reason: corrected milliseconds to microseconds