What I wrote is valid only for service's M16 and not from rifle range competitions - My own experience and of my colleagues was that, even at very short distance, we often saw shattering of the bullet - it may be indeed due to the fact that service bullets are, by nature, kept "in field conditions" and we focused on the wounds and didn't interest ourselves with the state of the barrels that fired the bullets or the state/age of the powder, though it makes indeed sense that this influences the bullet behavior.
This extract from a very nice Australian work "The Assessment of Bullet Wound Trauma Dynamics and the Potential Role of Anatomical Models" describes most perfectly what happens:
"5.56x45mmNATO:
At high impact velocities, all of these bullets tend to fragment and the penetrator (located in the nose) may become separated. Another feature of these bullets is that they consistently exhibit curved trajectories during gelatine impact testing because their centre of gravity is located to the rear of the bullet’s centre of drag (centre of pressure). The centre of drag is defined as the theoretical point (usually in the forebody) at which all drag forces can be said to be acting, which causes the bullet to turn as it starts to yaw during tissue or gelatine penetration (1, 80, 81). These types of bullets typically have full metal jackets comprised of copper nickel gilding metal, as well as a lead-antimony alloy core. Antimony is added to the lead to increase its stiffness for superior penetration (82).
The 5.56x45mmNATO bullet
typically exits the muzzle of a rifle with up to 6° of yaw at launch. As the bullet moves farther from the muzzle the gyroscopic stabilisation dampens the yaw, which gradually decreases to approximately 2° throughout the region of stable flight (pre-transonic — greater than Mach 1.2). This explains why close range wounds from this calibre are generally more destructive than long range wounds, because the bullet has become more stable over a greater range and has less remaining kinetic energy. Bullet fragmentation also decreases with increased shooting distance, and this explains why these bullets penetrate deeper at 100m range than they do at 3m (75, 83).
Another important feature of certain high velocity bullets like the 5.56x45mmNATO is their tendency to fragment. Fragmentation is the result of rapid yaw growth and is caused by a combination of forces that bend the projectile sufficiently to fracture the jacket (1, 54, 81).
All types of STANAG 4172, 5.56x45mmNATO bullets tend to flatten and break at the cannelure. This occurs because the cannelure is the weakest portion of the bullet jacket and stress forces focus on this area during maximum yaw. The bullet point tends to flatten but remains in one piece because of the structural integrity associated with the design (cone shape), but the rear section breaks into many fragments. This feature can cause major tissue disruption (84).
Reducing the barrel length of assault rifles such as the M16 in calibre 5.56x45mm, will also reduce the velocity generated. This can potentially reduce the effectiveness of this calibre depending on the target distance by decreasing the likelihood the bullet will fragment (47)."
the whole work can b found at
https://digital.library.adelaide.edu.au/dspace/bitstream/2440/99527/2/02whole.pdf - (5.56mm NATO is on page 38-39)
Doppz92, sorry to disagree but the aim of warfare is to win, not to kill - killing is just one of the ways to win - and disorganizing the enemy lines by encumbering them with flows of heavily injured patients was indeed one of the main rationales behind the development of the 5.56mm high velocity bullets - as it is much more "efficient" than simply killing individual soldiers - you neutralize in practice 4 to 5 soldiers in the chain of treatment/evacuation of such an injured patient with all what this means in terms of logistics. When the supply lines of an attacking force are "embottled" this means that the attack has to be terminated. Neither forget that NATO doctrine is a defensive doctrine, not a doctrine of aggression.