Twist rate performance is not determined by weight but by length, 62 grain M855 ball ammo will work fine in 1/9, the extra long 62 grain M856 800 meter tracer is the reason the military picked the 1/7 twist. Rule of thumb is that 1/9 is good for up to 70 grain, 1/8 is better for 70-77 grain, be aware though that accuracy of bulk grade M855 is not that great to begin with.