The issue with the M4 is that they shortened the barrel and that caused a reduction in velocity. The 5.56mm round relies on velocity to get the job done. It's got to do so, because the bullet weight is so light. Slow the 5.56mm down far enough and the bullet just doesn't have enough velocity to get the job done. They pretty much HAD to come up with something different ammunition wise because the 5.56mm is incompatible with what they need to do in the size gun they need to field.
The new rounds that have been developed for the platform like the various 6.8mm rounds use a heavier bullet at a slightly lower velocity than the 5.56mm, BUT, the heavier bullet maintains it's momentum because it is heavier and is more stable in flight because it is longer and more aerodynamic in design.
There's two ways to get penetration, mass and velocity. Heavy bullets retain their momentum longer (a body in motion tends to stay in motion) and light, really fast bullets like the 5.56mm have more velocity to shed, so it takes them longer to come to a stop. Look back into history for examples of this. The old big bore Sharps Buffalo gun loads maxed out at around only 1800fps velocity but threw 300gr (.45-70 carbine load) or heavier bullets. Bear in mind, these bullets were relatively soft lead alloys- no jackets or gaschecks- and weren't spire pointed like modern era designs but they were capable of shooting THROUGH large game animals like bison and hitting targets at what is still considered extreme ranges today (ie., Billy Dixon's open sight, 7/8 of a mile kill shot at Adobe Walls with a stock '74 Sharps .50-90 'Business Rifle').
Look at what J D Jarrett did in the development of the subsonic .300 Whisper loads and you'll see another extreme example of what I mean. The heaviest bullets (up to a 250gr bullet) are designed for very low drag and are fired at a subsonic muzzle velocity, yet are accurate at long ranges and capable of being suppressed to a level similar to subsonic pistol rounds.:blink:
