The only problem with this myth is that it is just that, a myth.
The U.S. military has never published any documents, requirements or doctrines stating a desire to adopt a rifle cartridge designed to only wound the enemy. Of course the military views wounding as better than no hit at all, and taking an enemy combatant out of the fight they view as a good thing. But they have never built a doctrine around the concept of wounding being the desired result of a gunshot wound.
The 5.56mm cartridge was designed to kill not wound or maim.
I believe the root of the myth comes from countless horror stories told by both civilians and Soldiers about the 5.56mm's failure to neutralize a target. In 2003 the U.S. Army conducted a study that found the 5.56mm was actually quite acceptable for combat duty. This is a quote from the study that I think carries a lot of weight.
In the end, “footpounds of energy” is misleading, “stopping power” is a myth, and the “oneshot drop” is a rare possibility dependent more on the statistics of hit placement than weapon and ammunition selection. Effectiveness ultimately equates to the potential of the weapons system to eliminate its target as a militarily relevant threat.I agree with this statement. Shot placement will be the largest deciding factor in how effective a gun shot would will be in terms of dispatching the enemy. The horror stories about the ineffectiveness of the 5.56mm can be traced back to either unsubstantiated rumors and myths or to poor shot placement.
It's also worth noting how the 5.56mm stacked up against the .308 in the testing. For CQB type combat the 5.56mm actually kept pace with the 7.62x51(.308) in terms of close range effectiveness.
It's interesting to note that when the U.S. military adopted the .308 to replace the 30-06, similar horror stories circulated. The .308 was deemed to be inferior to the 30-06 by many Soliders. Slowly these rumors faded, and their demise was hastened with the adoption of the 5.56x45mm only a few short years later.
The 5.56mm cartridge is a fine service rifle cartridge that excels at close to medium range combat. It is not well suited to long distance engagements which is why the U.S. military has moved back to the .308 for DMR's (Designated Marksman Rifle) in the wide open spaces of the Middle East. For a survival rifle you would be hard pressed to find a more suitable caliber than the 5.56x45mm.