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Originally, the M16A1 used a round known as the M193 ball round which fired a 55 gr projectile at around 3200 fps. It lacked the tungsten penetrator of the SS109 projectile found in the new M855 round. The M193 round is designed for use in M16A1s, not M16A2s. The M16A2 rifling twist is tighter (1:7") to stabilize the longer ss109 projectile and, as such, has a tendancy to twist the copper jacket off of M193 projectiles. M855 ball ammo, on the other hand, is designed for the M16A2 but will function in an M16A1 OK(or other AR style rifles with a slower twist). Modern AR clones typically compromise and use a 1:10" rate of twist.

I have read that the older M193 round was considered "inhumane" as it would fragmentate upon impact (due to its super high velocity). It directed its energy into the target as opposed to the M855 round which was designed to better penetrate a steel plate at 400 yards.

My point then is this, the 5.56 mm round is not low powered. The M193 projectile hits a soft target and explodes createing a tremendous wound channel. The M855 ball round was designed to penetrate a 10 gauge steel plate at 400 yards and it does so. That is not a characteristic of a low powered round either. Both rounds encompass the realization that the modern field of combat does not typically require one to regularly engage point targets beyond 500 meters. So, at the distances the round is designed to work at, the 5.56 mm round is effective. It is not as powerful as the 7.62 nato round of course so it will not penetrate vehicles or other hard cover nearly as reliably as the M80 ball round; on the other hand, a soldier can carry a lot more rounds which improves his chances of shooting more enemy soldiers. It's a trade off, as always.

I wound be interested to see a data table of "one shot drops" versus "one shot got up and kept shooting back" for the 5.56 round in Iraq and Afghanistan.