[rlmorel]

My understanding is limited, but that didn’t seem like a meaningful explanation.

I assume any round will begin to tumble as its energy is expended near the end of flight, but once it starts doing that, I assume it will lose any accuracy.

Anyone with better knowledge on this who can explain it (or perhaps you can, ManHunter?)

[Robert A Cook PE]

I think the tumbling refers to the bullet path only after hitting a ballistic target (the body mass). In air, it spins cleanly and evenly from the rifling in the barrel. Don't know exactly what would exaggerate tumbling after impact - maybe a hollow spot behind the nose.