Forgive me for not being exact, as it's been a bit on a materials engineering principal I had to learn to grasp the conversation. But basically the understanding is that rule of thumb, reverb energy moves through barrels at roughly 18,000fps and with minor variations, it's relatively consistent with the materials that are suitable for barrel construction.
That said, the barrel primarily flexes in an up and downward motion. The goal of the "timed reload," is to exit the barrel on the pauses. That is, think of a teeter totter, when the transition occurs from the up to downward pause there is a momentary pause, motion stops as momentum switches from the upward to downward flex cycle. Another pause, of course, occurs at the down to up transition. The goal is for the projectile to exit the barrel at this pause, as more or less the barrel is in the "same," location. Where as, by firing whilst the barrel is in motion you have the inconsistency of powder charges, projectile shape, case weight, etc (commonly expected issues that effect even the most perfect bench rest rifles of course) as well as the fact that the barrel is in motion, which transitions some of the momentum into a "flinging," effect. While firing on the pauses, the barrels energy of motion isn't transferred to the projectile, because the barrel is no longer in motion.
The goal is to fire on the upward pause, though the lower pause will also work for accuracy. But you may as we'll take advantage of the extra elevation if for no other reason. So as Ed explained, the barrel length matters less, as they'll all flex, it's just about finding a projectile that fires on your particular pause. This is the reason why your load may shoot lights out in your rifle, and shoot like utter crap in your buddies relatively similar setup.