Originally Posted by Mikecr
I'm sure there is a rational reason.
I've never seen, or met anyone who has seen, DECREASING velocity with MORE powder.
The velocities always climbed, and predictably via Quickload.
I have seen this happen. It has nothing to do with a drop in bullet velocity with an increased load. Follow this explanation:
As the load increases the muzzle pressure and muzzle blast increases. The propellant gas released as the bullet clears the muzzle has a considerably higher (molecular) velocity than the bullet and will pass the bullet for some distance. That gas can trigger the first screen of the chronograph prematurely causing the time between the triggering of the first screen and the second screen to be longer. That is interpreted by the chronograph as a lower velocity.
I'd recommend repeating the test exactly as before, but with the chronograph screens set a couple feet further from the muzzle. I'd bet the measured velocity drop goes away and shows normal increasing velocities.