Greg Hamilton recently posted the following to the email list regarding the terminal ballistics of rifle bullets at different velocities.

With rifle bullets you have:

1) A velocity at which a given bullet will fragment.
2) A velocity where it is stable through tissue but not going fast enough to fragment.
3) A velocity where it is so slow it is very unstable.

All three depend greatly on what the bullet is going through specifically, how far it travels, how many different mediums it passes through, spin rate, etc. It is number 2 that is most affected by these factors.

Many 5.56 bullets when they get down in the low 2000fps range have weak wounding (don’t fool yourself into thinking anything as weak as .22LR) but there will be a lower velocity where the bullet will become unstable very quickly and turn over producing more wounding then if it was going faster and did not turn over. The 300 whisper 220gr subsonic is an example of this, the bullet becomes unstable almost instantly when it enters tissue and really rips stuff up in a non-intuitive way.

There is also a velocity where the bullet will turn over and then break and one where it will not (once again, this depends on a lot of factors) the perfect subsonic round will become instantly unstable, turn over and break into fragments. Many bullets when traveling at subsonic velocities will turn over, but not break as the bullet construction is too strong. I’m sure low velocity 5.56 would do the same, but I have never shot the subsonic stuff into any living thing so I can’t tell you.