Originally Posted by Bart B
As I've sometimes done, yes, you misunderstood. Welcome to the club.
I've never heard of changing seating depth to change muzzle velocity such that bullets exit at different muzzle angles.
But the ideal place for bullets to leave in the barrel's vertical whip cycle is just before the bore axis at the muzzle's risen to its greatest angle. Slower bullets typcally have a bit longer barrel time than faster ones so they'll leave later when the muzzle axis angle's a bit higher compensating for their greater drop. Faster ones leave sooner and at lower angles. I don't know if anybody's measured this in recent years using accelerometers on the barrel connected to test equipment that shows where the bore axis points when the bullet leaves. There's been calculations made based on accepted facts of metalurgy behaviour and vibration analysis. And theories abound all over this planet about this subject which have never been verified by actual measurements.
Without a doubt, it could in theory happen.
Even if someone proved that it does occur, then how would you prove it is occurring as part of your load development? And, how sensitive would this be to changes in seating depth, case capacity, temp, MV, etc?
Ultimately, you have to look at group sizes at long range. The more groups you record in varying conditions, the better your confidence will be.
old brass, new brass, whatever... just don't change something last minute and go long range hunting