Interestingly, the Rifleman also features a story on the 50th anniversary of the adoption of the "little black rifle" — the M16, and, very fairly, includes some of the horror stories associated with its use in Vietnam.
Regarding Kyle's use of the 5.56 in the M4 variation during the battle for Fallujah, the article (p. 40) says:
While supporting the battle, Kyle reported, he averaged two or three enemy kills per day, for a total of 40. Several times he shot insurgents at close range with 5.56 rounds and they did not halt, causing him to question the cartridge's effectiveness. That ineffectiveness may have been caused by drugs. "In Fallujah we learned the bad guys were on dope," he said. "The Americans gathering [enemy] bodies for burial found track [needle] marks on the arms of enemy dead." U.S. forces also found burnt spoons from cooking heroin, syringes, and black tar heroin, he said.
I will give you the fact that the attackers were hopped up on drugs, but this is an expert and experienced marksman shooting at close range. He had a one-shot, confirmed kill at 2100 yards — a mile and a half, with his sniper rifle (not sure, but I suspect it was the .338). I would guess that his rounds were going into the vitals of the those attackers. Heroin or no heroin, he would have been better off with something that made a bigger hole and punched harder.
Once again, the .223 is an excellent hunting round with flat trajectory, accuracy, and potency within its limits. It is a reliable 250-yard varmint round, and I have no qualms using it on deer within 150 yards. It's not a bad zombie gun since you have to use head shots anyway. For defending the homestead, I think it can serve well enough, provided non-varmint, expanding ammunition is used. If your home defense rifle is an AR-type or Mini-14 or similar semi-auto in 5.56, practice double-tapping everything, every time. The experts can drone on all they like about the advances in technology. Most of the time, life-and-death confrontations take place at distances measured in feet, in a few seconds time, with the total number of rounds fired by all parties averaging about five. Magazine capacity, range, and mechanical accuracy are all secondary to reliability and skill in the preponderance of these situations.
I still like my .30-30 and my shotgun.