I surrender. That is my opinion, and although I've heard it said by many people, I can't prove it as fact. That the 5.56 is less likely to cause a first round kill than a 7.62 is a fact, however, which I've seen in practice. In fact (

), a year or 2 ago, the Army was studying a more "robust" round to replace the 5.56. a 6.8 mm round, in the XM-8 rifle, and came close to accepting it. Instead, they've reopened their study and may be going another way. Bottom line is, the 5.56 round isn't as lethal as the 7.62 round it replaced, and it doesn't have the same penetration through building materials. The Army has re-issued an ass load of M-14's for use in Iraq and Afghanistan for those reasons (they've also send a bunch of old M113 APC's to Iraq, because the M-2 Bradleys and Strykers are too big to be as manueverable in urban warfare).
While I can't prove the design specifications of the 5.56 round, I did find this info that supports my statement:
2. APPLICATIONS OF WOUND BALLISTICS DATA
At many points in the Army Research, Development, and Acquisition (RDA)
cycle, the requirement arises for quantitative comparisors of weapon performance
among competing candidates. Development of these quantitative comparisons is
the business of the various agencies in the Army assessment, evaluation, and
analysis communities; the comparisons are used to support major milestone
decisions throughout the RDA process. For weapons which are primarily
designed for an antipersonnel role, one quantitative comparison of interest is the
ability of the weapon to degrade a soldier's effectiveness in performing military
tasks.
In any armed conflict, the objective is to neutralize the opposing force. While
killing an enemy soldier certainly accomplishes this, incapacitating him (i.e.,
destroying or degrading his ability to complete his tactical mission) achieves the
same goal and places an additiona; burden on the opponent's medical and
logistical resources. Tt is actually the weapon's ability to incapacitate, not wound
severity nor killing potential, that is of interest to weapon designers. . .
This came from a 1991 paper put out by the U.S. Army Ballistics Research Laboratory. It can be found at
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA240295&Location=U2&doc=GetTRDoc.pdf . The quoted language can be found at page 1 (not page i).