10 twist 300RUMs using US869 at near max loads will work fine on 240SMK's out to about 1500, so I'm sure you're fine with a 9 twist for 210's. That's the word from guys who've shot that far anyway. I've only shot to just over 1000 (getting there...) and when did my part measuring/reading canyon winds, the 240's were doing minute of elk. Might talk to Kirby, but that may even stabilize the 265 AT RBBT Wildcat--1/8 might be better for the 265 though. My Sierra bullet chart reccomends 1/10 for the 210's and 1/9 for the 240's. Good luck.