Fourteen inch twist has been the standard for the 222 from day one. The heaviest bullet these guns will shoot accurately, I have found, is the sierra 63 grain semi point. These bullets are shorter than other 60 grain bullets which is the reason they stabilize well in 14" twist. Years ago I put togather a 222 with a 15" twist Douglas barrel for 40 gr. bullets and it turned out to be one of my best varmint guns. Never shot any heavier bullets in it although I think it would have done well with 50 grainers. I'm sure 14 twist would have worked just as good but I think the 15"gave me a little more velocity. Probably damn little!