picher: "It's believed that the ideal situation for a load is when the bullet exits the barrel slightly before the muzzle reaches the maximum vertical amplitude of it's vibration, when it's nearly stopped moving in that direction. That's when minor variations in pressure/velocity don't make as much of a difference in vertical displacement of projectiles (POI)."
I have seen this theory before and seen some interesting graphics on the subject. However it strikes me that the vibration amplitudes would be 360 degrees around the barrel rather than just vertical, so the the variation may be in any direction. This is all theoretical so it's really not of much consequence to me. I just load to get the best accuracy. Doesn't matter to me about the gyrations of the barrel. But it is an interesting theory.
Cogito, ergo armatum sum