or the bblIt's not over spinning.
Gotta be the bullet
Probably overheated from the combination of twist and muzzle velocity and or the barrel could be a little rough or fouled or all of the above. And if it it was just one bullet it might have just been a bad one. It definitely needs some diagnostic.. My suspicion is if it was pushed a little harder little grey puffs will appear..Too slow:then,?
Same findings here.in my experience, some bullets (sierra, berger) are more tolerant of high rpm (>300K), while others (hornady) are not. I've had overspun bullets keyhole like the OP's. slowed them down and "most" of the problem went away with only an occasional keyhole. Solved the problem by changing bullets.
That's 391Krpms and all the energy to get there..3,800 fps with a 115 VLDs
Bullets don't always "disintegrate" when the core and/ore jacket gets damaged form running them to hard, ill post a picture later on when I get home from a bullet i found with a jacket "failure" that still tumbled to the target in more then less one piece and I suspect that might be what's happening with you. As to "over spinning" them no not unless they "pop" or the jacket gets damaged , i have shot 70gr bullets out of big case 8 twist 6mms at crazy speeds and they will all group till they don't make it to the target