Optimum twist for 150-168's will be 1-13x. If you think you will ever shoot anything heavier, then go tighter (12x for 180's and 190's, 11x for 200, 210's and up to the 220 and 10x for the 240). If you KNOW all you ever want is up to 168, go with what is optimum which is 13x. I am one that believes in spinning a bullet to a point of proper stability and nothing to little more. My target SF is 1.4-1.75. This offers the best accuracy, minimizes spin drift and maximizes long term BC potential. Most shooters agree that up to 2.0 is OK. Throw a 10x in that 300 and shoot 168's you will be approaching 3.0. Keep the 10x for the 220 or 240's.