As a general rule, for the fast 1:9 twist, heavier bullets are better (55 grains and up) though some folks report good accuracy with lighter bullets as well in their particular rig. For the 300, you'll have to try the loads you plan on using. In either case, you may find good loads right off the bat or have to experiment and determine what your rifle(s) like. Rifles can be individualistic and a load that works in one, may not work in another.