"I'd be most curious as to how the powder makes it's way by truck/train/ship from the plant to the distributor to the store to the consumer's home without being "tumbled"."
That one is easy to answer. The transportation method involves vibration that is both damped and random. The tumbler is a fixed frequency forcing function.
"Not to mention the hundreds of millions of rounds that have been transported in every way shape and form for war-time."
Same thing said twice. Damped and random is still damped and random.
"After the experiments I've seen posted online, wherein some loaded ammo was tumbled for DAYS and then fired over a chrony, with ZERO change from the control batch..."
Now you have reached the heart of this debate. What potential problem do we anticipate from tumbling?
The tumbler is going to be a fixed frequency. That can have structural effects. Resonance is unlikely due to the grain size and composition, so the powder grains will not shatter like glass when the tumbler hits "high C". That leaves the mechanical motion induced by the tumbler. If the powder is so constrained by the container (case volume remaining after the bullet has been seated) then there is little or no movement. Motion = friction. No movement means no friction, means the powder is not self-abrasive. The powder would be self-abrasive if there is plenty of remaining case volume to allow movement.
So for an experiment to be viable, the tumbling would have to allow for the powder to move against itself within the case and that amount of movement would have to be quantifiable and repeatable. In addition, where grain size/shape is a factor in burn rate control, that would imply a separate experiment for each powder type.
"I have no concerns whatsoever."
Lets try these:
1. If the powder experiences abrasive friction, it will wear down. The only questions with a well defined experiment would be time and abrasion rate. How much time ? How much empty case space ?
2. In well controlled experiments, expect and try to anticipate the unexpected. I have seen no concern expressed for other components, such as the primer.
The powder will be more or less contained but not constrained by the case. The primer compound is constrained by the primer cup, is constrained by the case, and the constraining material has very different physical properties from the material it constrains. I am saying that the possibility exists that the priming compound, in a fixed frequency environment is the one item in the case that I would expect the unexpected because of the nature of its constraint.
"Here's a link to what I consider an exhaustive test of this issue on AR15.com a while ago."
Thanks for making my point for me. Excellent photo work. That fellow has the ability to resolve this issue if he could apply his photo doc talent towards characterizing how much powder abrasion was applied to the powder. While I agree that the powder shows little abrasion, I also believe it experienced NO movement. What happens when the powder is free to move around more ?