Studies have shown that the number of casualties per round fired has been dropping ever since firearms were invented, even though weapons are much more accurate now than they used to be. Most men don't aim, they just fire in the general direction of the enemy. Eventually, the decision was made among American forces (since we have such overwhelming supply and logistics superiority to all our enemies) to simply overwhelm the battlefield with "supressing fire" until the artillery or gunships show up. In other words, we shoot just to keep the other guy's head down.

Did you ever hear of "mad minutes"? It was a tactic which had fire base troops simply fire out into the darkness at random, thousands of rounds, at unexpected times during the night, simply to discourage the enemy outside the perimeter from organizing an infiltration or reconaissance or maneuvering into an assault formation.

A mass-produced standard issue WWI, late 19th/early 20th century rifle and cartridge was accurate to almost 500 yards. They even had sights which allowed massed fire to be used as indirect artillery over longer distances. Trench warfare ranges demanded highly accurate weapons, with very, powerful cartridges capable of precise fire and heavy damage at those ranges.

In WWII the Germans invented the assault rifle for use in the in-close environment of urban warfare to counter the machine pistols favored by Soviets, the Brits developed the jungle carbine and we the M-1 carbine, with smaller, lighter cartridges (so you could carry more of them) and the trend was to automatic fire and less fire discipline. The Russians followed up with the AK-47, the most effective and practical mass-produced infantry weapon of all time. They were cheap, easy to build, use and maintain, and indestructible under realistic field conditions. You're crazy if you arm your troops with anything else, unless you're richer than Croesus, like we used to be.

The ability to supply troops easily by air led to a proliferation of different weapons and calibers often carried by the same infantry unit. Standardization and interchangeability were no longer important, men were dropped and retrieved by chopper, and supplied by air. You could afford to be wasteful.

In the WWII US infantry fire team, every man had a .30 cal rifle, and they carried a Browning Automatic Rifle, also .30 caliber. At the platoon level, there was .30 cal Light Machine Gun fire support. All the same caliber! Ease of manufature and supply. A few troopers might carry a Thompson or a grease gun for specialized close-in work, and they used officer's sidearm .45 ammo.. Two calibers supplied everybody up to the company level! Even the .45 brass casing was the sawed off end of a .30 cal rifle casing, so the same machine tooling could be used to manufacture both rounds.

But the doctrine is different today. It is wasteful, and probably cowardly, but it works. The grunts are not aiming any more, it is too dangerous. They just stick out their rifles from behind cover and spray in the general direction of Charlie. You don't need accurate, precise weapons except for your elite troops, like snipers.