In Vietnam, it became apparent that it is a massive overkill both in terms of recoil and weight. The US transitioned to the 5.56 mm. The Soviet 7.62 x 39 was better in that regard. It was sufficiently lethal since killing horses from 1000 yards wasn't important, and was controllable. But it had problems with magazine capacity and external ballistics because of its low slenderness and speed. The Soviets developed 5.45 x 39 mm. The US kept the 7.62 x 51 in service as a machine gun round since the 5.56 wasn't enough and there was a lot of 7.62 in the inventory. Soviets never ditched away the 7.62 x 54.
This is how it’s normally restated with the AR15 emerging from nowhere as if Eugene Stoner appeared from a time machine or something however it’s not so clear cut.
In the same period of the 1950s the Us Army did trial alternatives to the T44(M14) including the (T48) FN FAL and hand built Armalite AR10 all still in 7.62x51mm. This was 1957.
As this is happening inside the same Army Ordinance board the US Army is studying a number of alternative technologies. Among them was Project Salvo. Salvo was looking into duplex rounds, flechetee and multiple barrels, crazy stuff they were highly interested in a .224 and .25 cartridges. Though they didn’t come to anything they were looking at materials to which is likely where interest in Armalite come. Salvo had concluded that a lighter weight, rapid fire highly accurate rifle firing a small caliber high velocity round was the way forward.
In 1958 after the AR10 had been dropped from the M14 trails Stoner and Armalite were contracted by the army to rebuild the AR10 to chamber one of the cartridges that had been developed from this. .223 Remington.
Stoner and co set to work and developed the AR15. It was submitted to trials by the Army in 1959 whom apparently like it, but it was vetoed by General Taylor whom wanted a universal cartridge.
However the Armalite Fairchild company was in financial difficulties and sold the rights to Colt Manufacturing in 1959.
Colt modified the design for manufacture and offered it as a product. They focused sales on Asia, making first sales to Malaya.
The USAF got interested in 1960 after seeing the rifle and impressed with it SAC commander Curtis “Big Cigar” Lemay ordered them for security forces around SAC bases. The Army was still experimenting with them with favorable results.significant improvements on accuracy and the ability to get soldier to expert marksmen badges vs M14.
Yet an internal conflict inside the DOD was emerging as the USAF wanted them and elements of the Army wanted them but Army command didn’t. Made even worse by the fact that aspects of the DOD and Army had started fielding them in Vietnam. Where they were praised. In 1963 the decision was made and the Army adopted AR15 as the new rifle.
However at this point the .223 cartridge had only really been made for a rifle not a MG. So the US maintained the M60. The closest to a .223 Remington Mg was developed by Stoner and the brain trust of Armalite who were then working for Cadillac Gage in the early 1960s the Stoner 63 modular weapons system. However that system was highly highly sophisticated and modular. Though it would see deployment in Vietnam it would not be as successful. Though attempts would be made to adopt M16 into LMGs they also we’re pretty much a dead end. It wasn’t until after the adoption of the Belgian SS109 5.56x45mm in the 1980s that a successful 5.56x45mm LMG the FN Minimi would appear. The M14 though never completely left the field during Vietnam it was found that though good for a general purpose rifle a Marksman/Sharpshooter or Sniper rifle was needed. The USMC has retained some Sniper capabilities with Marines building Sniper rifles off of Commercial hunting rifles. The Army created the XM21 from M14 rifles. 7.62x51mm has remained and is still in wide issue globally particularly in nations whose armies expect to face longer range engagements. India, Pakistan, Turkey, Iran all maintain it prominently for potential conflicts in high altitude areas where the longer range aspects can be advantageous. A number of other nations have looked to it as a DMR rifle caliber or as they deem it better suited to engaging body armor. Though vehicles maybe increasingly armored against it the 7.62x51mm can still be cost effective well being anti personnel effective for a moderate investment vs even armored infantry.
The Soviets 7.62x39mm round and AK series came from a different set of doctrine and experience both were developed starting in the Second World War. The M43 cartridge was developed for the RPD in 1943 and production started in 1944. Followed by the SKS in 1945 Then AK47 would come soon after in 1946. The Soviets based the AK doctrine after seeing the German use of the STG44 and it’s forerunners. Based on close quarters battles in urban environments. It started its official role in the Soviet army as a Submachine gun counterpart to the SKS. Eventually the AK superseded the SKS. Like the US though the Soviets concluded that they needed a heavier MG and Sharpshooter rifle along with these so the PK and SVD were created retaining the 7.62x54R dating back to 1891.
the Soviets eventually adopted the 5.45x39mm but where the US got a major weight savings from 9 pound M14 to 7 pound M16A1. The AR used a lot of new materials that the AK never got aluminum receivers and polymer furniture. The AK got its own polymers furniture down the line but the receiver is all steel. The AK series is to this day an World war 2 rifle with modern bits bolted on. M14 was the same AR15 is a space age rifle. The Russians never really phased out 7.62x39mm and even their newest supposed replacement for the AKM and AK74 are in both calibre
The PLA started off on its 5.8mm far later than any of the above cartridges. Long long after the now over century old 7.62x54R or 30.06. Decades after .30 carbine, 7.62x51mm, .25,.224,.223 and 5.45x39mm. The closest in era would be the 5.56x45mm NATO SS109. As such the 5.8x42mm was developed from lessons learned by these cartridges. The Chinese built their own doctrines around it. Though it seems that as they pregressed they found that some of the aspects were not as effective as they would like. The latest arms shows clearly show that the 5.8x28mm is a dead end as far as the PLA is concerned. They redrafted their 5.8x42mm for a third generation in the QBZ191 carbine as they decided to follow the M4 configuration for their replacement of the QBZ95 series.
