I understand it is a very nasty round, wound wise.
Isn't the SS109 round designed to break into 3 parts? But the velocity has to be up around 2800fps.
The short barrels on the M4 reduce the range that that velocity can be maintained. Less than 100 yds.
The longer barreled M16 16-20") increase the lethal range.
Think that's what I've read.
Don't think so, as I recall (we know how that works when you get older) the SS109 was designed to better penetrate helmets. Apparently the older 5.56 round had a tendency to bounce off at extended range, no surprise there.
The M855 62gr FMJ has the SS109-type bullet, but is designed to fragment very similar to the older M193 55gr FMJ. As the velocity drops, both rounds fragments less and less. The M855 round starts off a few hundred FPS slower than the older M193 round, so it's not as good of a round close in. However, the heavier round holds its range better.
The "Black Tip" M995 round is the true armor piercing round, with several components combined into the bullet. It's designed to penetrate armor and not specifically fragment like the standard rounds. It's an expensive round and rarely issued.
Here is a GREAT site on 5.56mm ammo.:
www.ammo-oracle.com