Like J Baker, I've had the grey mist occur with my .22-250 with 40 grain Nosler BTs at about 4200 fps and it only had a 1:14 twist.
Never had it happen with 50 to 52 grain bullets with my .22-250, although I never even approached that velocity with the bullets above 40 grains.

I've never seen a grey mist cloud with a 1:9 twist .223 even with 35 grain bullets.

For some data on 77 grain bullet performance in a 1:9 twist barrel:
https://www.savageshooters.com/showt...9-twist-barrel