“I had to re-read the developmental history of the AR series, and indeed the 10, @7.62 came first.” Seaplaner, post 18]
“yup, the idea of a small rifle round for general issue may not have even been a gleam in the military’s eye yet” [old-ager, post 21]
“Okay. Here we go. Is the main diff between the two caliber?” [murrie, post 35]
By 1957, when the M14 was officially adopted as the new standard-issue US service rifle, Army Ordnance had already initiated their Small-Caliber High-Velocity (SCHV) program.
By the 1950s, World War Two small arms were still standard issue, but increasingly obsolescent. There were many disagreements inside the Army hierarchy, on the direction future small arms development ought to take. Some favored a reduced-power cartridge firing a bullet of standard diameter but reduced weight: the classic assault-rifle round as embodied in 7.92x33mm Kurz (German), or 7.62x39mm o1943g (Soviet); others insisted that only a “full-power” cartridge was acceptable. The full-power advocates won, and 7.62x51mm was adopted as the NATO standard rifle cartridge in 1954. Ballistic performance at the muzzle was almost identical to the US 30-06 military loading (30M2 was the nomenclature for the round used during World War Two and through most of the 1950s).
In 1950, Remington introduced the 222 Remington cartridge to the sporting world, producing performance and accuracy all out of proportion to its size. The military took an interest and SCHV was started up.
While designers struggled to produce a select-fire shoulder arm firing 7.62 NATO, SCHV experimented with more radical concepts, including an M1 Garand firing a 22 cal round necked down from 30-06, and a two-barrel piece based on the FAL.
Most attention focused on the 7.62 NATO rifle competition, which included Belgian and Spanish rifles. ArmaLite, an engineering/development company, entered its own rifle designed by Eugene Stoner - the AR-10. It arrived too late to become a serious competitor and experienced durability difficulties (Stoner, a WW2 combat vet of USMC, favored the 7.62mm in general). Under irregular circumstances that are still argued over to this day, the M14 was selected over the closest competitor, FN’s FAL.
Meanwhile, Winchester developed a 22-cal rifle based on the M1 Carbine, slightly enlarged, for the SCHV program. Eugene Stoner left ArmaLite, but his erstwhile colleagues redesigned the AR-10 to handle the new, small 22 centerfire rounds. Winchester’s rifle failed the accuracy tests and they withdrew, disheartened by what they saw as an obsession on the part of Ordnance officers with “hi-tech” rifles made of space-age materials like aluminum and plastic. What eventually became the M16 performed well in tests, and a few rifles were sent to Southeast Asia, where US advisers were already at work. It performed very well in action against the Communist guerrillas; more rifles were demanded.
At this point the US Air Force voiced interest in the “pre-M16,” which ArmaLite had named the AR-15. USAF was displeased because the adoption of the M14 meant it would be deprived of its M2 Carbines, widely used as a guard gun in air base security: the M14 was overpowered and too heavy. The Air Force contracted with Colt’s ( which had bought a production license from ArmaLite, and intellectual-property rights to the name “AR-15”) for 20,000 rifles.
The US Army stepped in to halt the contract, citing its authority (granted back before World War One) as “executive agent” for small arms. Rifles initially produced went to Southeast Asia; meanwhile, SecDef Robert Strange McNamara terminated the M14 production after numerous quality control problems and delivery delays. The still-experimental “pre-M16” was issued in large numbers to ground forces, leading to various modifications and eventually to adoption as the official US rifle.
USAF did gets its M16s eventually. Many are still in use by security forces.
SCHV was covered in detail in the print edition of American Rifleman magazine, about two or three years ago. The USSR began developing its own small-bore cartridge in the mid-1950s; this was not known until many years later. The result was 5.45x39, adopted in 1974.
In terms of size and performance, each round of 5.56mm NATO weighs only half what 7.62mm NATO does. The original bullet for 5.56mm (introduced as 223 Remington in the early 1960s) weighs about 1/3 of what a 7.62 NATO bullet weighs, and the round uses a bit more than half the propellant. Effective range was originally about 2/3 that of 7.62 NATO. Recoil impulse is far less than that of 7.62 NATO, rendering the M16 controllable on full-auto, which the M14 is not.
Interestingly, back in my USAF days in the '60s, I never got to fire an M-16 as they were always prioritized to 'Nam or to others who had valid needs.
The AF did most of its annual qualifications using the ancient M-1 carbine, a firearm that I never found to be very accurate.
One year, I did get to qualify on the M-2 carbine including full auto training. (Full auto training was fun, but its only value to me was that it showed that every Hollywood director gets it completely wrong.)