An infantryman can carry a great deal more 5.56mm ammunition than he could 7.62mm NATO (.308 Win.) or .243 Winchester. Weight is the issue, and more rounds are always better if one becomes decisively engaged. When on patrol, my standard ammunition load was 20 thirty-round magazines, plus another 280 rounds in bandoleers or stripper clips. Try throwing 880 rounds of .243 ammo in a daypack and carrying it around for a day--you'll soon get the idea. Infantrymen armed with a .243 caliber rifle would be forced to carry less ammunition (although they probably could do more harm with the lesser amount). Also, more 5.56mm rounds can be carried per truck or helicopter load on re-supply runs.

To the Army, the cost of ammunition per round is important. While not significant in small numbers, it becomes apparent when you look at the big picture. One must include the cost of developing and maintaining proficiency with individual weapons. The 5.56mm gives the taxpayer more bangs for the buck.

Also consider the number of .243 cases that can be produced from 100 tons of brass versus the number of 5.56mm cases, and do the same for powder, lead, and gilding metal. While none of these are in short supply, they do cost money.

Battles are usually decided by maneuver. On the modern battlefield the lethality of one's rifle is of little concern. It is only one weapon of many available, and usually not the first choice. Much better to fix your enemy in place by launching grenades at him while you concentrate indirect fire (artillery/mortars) on his position. Rifles work better for clean up. Firing well aimed shots at enemy personnel is not quite the same as fire and maneuver up a hill into an enemy defensive position. The killing efficiency of an infantryman has much more to do with his training, leadership, personal equipment, available fire support, and his mental outlook than the caliber of his rifle.

One final point in favor of the 5.56mm--I can tell you from first hand experience that the round is considerably more lethal at close range (where end-game battle often winds up) than at long range. I would suffer a significant amount of inconvenience to avoid being hit with one at a range of less than 50 meters. A battle begun at 200 meters can turn into a 50 meter slugfest in as little as five minutes. Rather unnerving, but nonetheless it happens.

On the other hand, I agree more than you might think with the point of view expressed in the article "New .243 Service Rifle Cartridge, A Proposal." The Army/USMC could benefit greatly from the addition of a 3-12x scoped M-14 type weapon chambered for the .243 cartridge (perhaps two per squad). I say the M-14 because it is already chambered for 7.62mm NATO, and the .243 case is simply the 7.62mm NATO necked down to .24 caliber. This conversion would only require re-barreling as the two cartridges operate at about the same pressure.

In service I had the opportunity to use the M-21 sniper system based on an accurized M-14 rifle, and found it to be a very fine weapon. Such a weapon would be quite handy in the hands of trained shooters, but probably not so much in the hands of the average grunt. A couple of trained shooters per squad with an effective .243 weapon would increase the combat capability of the squad.