Jimro, here's a link to the theory behind OCW:
It's all about the shock wave making the muzzle bigger and that's believed to not be a good time for the bullet to leave.
He's listed the speed of sound in steel at over 18,000 fps but others say its about 13,000 in 416R stainless steel used in rifle barrels. He also states that shock wave makes 4 or 5 round trips before the bullet leaves the barrel; his math ain't so good.
Rifle barrels, when screwed into a receiver bolted in a stock, whip and vibrate much like a fishing pole held in ones hand when it's flexed. Its back end doesn't move much at all but its front end does. Only when the barrel's free at both ends does it wiggle in somewhat of a figure 8 pattern. Best on-line examples of exactly how it whips are at:
A barrel's resonant frequency it vibrates at will be different when screwed into a receiver (one end fixed) than when free at both ends. It's higher when free, lower when one end's fixed.
While Varmint Al's numbers for the bullet's barrel time are not correct, his barrel whipping wave shapes are very accurate. Other links on that page show how bullets leaving as the muzzle axis swings up and is close to the top are at a good place for the bullets to leave at.
But I don't understand your comment
that loads that have the same barrel time and different velocities can both group well, or you find distinct "accuracy nodes" during a load workup (For a 308 normally around 2,400 fps, again at 2550 fps, and again at 2650 fps, and 2750 fps (with a 168 or 175 bullet), and again at 2900 to 3000 fps for long barrels with 155 gr bullets).