Some of the current debate about the effectiveness of the M4 is swirling around the “stopping power” of the cartridge and the reliability of the weapon system. Digging through my archives, I found some comments from John Holschen regarding the effectiveness of the cartridge:
The stopping power “problem” is based on the misconception that there exists a hand-held firearm which can instantly terminate hostile behavior (reliably and repeatedly).

How quickly can we expect a bullet to terminate hostile activity; 1-3 seconds, 3-5 seconds, 5-10 seconds? How fast is fast enough?

A couple of years ago I reviewed an LE shooting where a BG was hit 12 times with an AR at a range of 9-12 yds.

* 10 rounds struck his torso producing fatal damage to his liver, spleen, heart and both lungs.
* 1 round struck his right femur fracturing same (and starting his fall toward the ground.)
* 1 round entered through his left eye and destroyed a significant portion of his brain (this was the last shot according to forensics but they noted the BG was already falling at the time this round hit him.)
* The shooting was captured on both video and (separate) audio recordings. The elapsed time from the LEO’s first shot to his 15th shot (total rounds fired) was just under 5 seconds.
* During those 5 seconds the BG continued to fight, firing 6 rounds from a .357 revolver.

Having viewed the autopsy photos of the BG’s heart, lungs, liver and spleen I can tell you that the contents of his torso were pretty much trashed. I just don’t believe that some other caliber would have made a significant difference in the amount of damage. Trashed is trashed.

* The stopping power “problem” is based on the misconception that there exists a hand-held firearm which can instantly terminate hostile behavior (reliably and repeatedly).
* Animals, human and otherwise, can suffer massive fatal wounding and keep performing meaningful activities for some period of time before expiring.
* Multiple torso hits and/or more potent cartridges make it more likely that fatal damage is inflicted but do not reliably reduce the time until incapacitation.
* Hits to the CNS are more likely to achieve a quicker incapacitation but are difficult to accomplish in many situations.

Believing that the 5.56 “stopping power problem” is solved by a different bullet and/or cartridge is likely delusional in my opinion.

As an aside, I have spoken with old-timers soldiers who expressed a poor perception of the 7.62 NATO cartridge as a “man-stopper” when our military transitioned from the M1 Garand (.30-06 cal.) to the M14 (7.62mm). I expect if you go back far enough you will find soldiers who felt the same when they had to give up their .45-70 Trap door Springfields.