Ton of info on the net but IIRC the 55 grain bullet was intended for the older 1 in 12 twist rate on the M16A1 and AR15 SP1 and
the 1 in 7, 8 or 9 twist rates are supposed to work better on the 62 grain round.

Be aware M855 "green tip" ammo has a tungsten steel core penetrator inside for armor piercing, not recommended for indoor ranges or on steel plate targets.

There was an interesting article in the American Rifleman a few months back that compare the M855 to the new M855A1 round,
IIRC they pulled and bisected a bunch of the older M855 bullets which they found lacked consistency in the seating of the core etc.

My guess is you should get better accuracy from lead core FMJ 62 grain commercial ammo than green tip or 55 grain .