well.... this one is rife with answers, all proly correct.
If you wanna go with origional military thoughts and designs, the the 55 grains is what you want. Origional thinking was, to be able to carry lots of rounds, most fighting will be up close, and will need lots of ammo poured in to the situation. the 55 grn bullet in a 1/12 or 1/14 twist bbl is highly unstable, and not very accurate, which is fine out to 100 yds, or in a raid or ambush. the high unstable rate is good, because, when it hits tissue, and with the slower twist rate, it is supposed to immediately yaw, and gimble, thereby injuring your typical commie.. er ...vc... ahh, you know, the enemy.
The thinking of the Generals at the time was , if it kills them, fine, if it does not, it takes two or more of their buddies to carry him away, which is very good for us.