But without the cannelure the bullet will tumble but not fragment.
Which is why all .225/5.56 is not created equal.
If you want the best performance from a .223/5.56, you need the cannelure. The bullet tumbles and as it reaches 90 degrees, splits into two pieces at the cannelure. This is part of what makes this round such a great stopper.
I believe it might also have to do with the type of jacket, but IIRC the cannelure is what causes the fracture.
I'm just reiterating what I've learned over at
http://www.AR15.com. Go to the Ammo forum and there's a FAQ there that is great.
Correct me if I'm wrong, but I won't be buying anymore ammo that is not M193 spec.