I recently "upgraded" my launching platform for .308 ammo and took a second to pull a couple of other rounds out of the ammo locker to look at side-by-side..
From left to right they are the NATO 5.56x45, Russian 7.62x39 and the NATO 7.62x51 (.308).
All three of these rounds share a common developmental origin of being developed after WW2 based on the experiences that the parent countries had experienced. There was a study done after WW2 by the US that showed that most soldiers only engaged targets that they though they would be able to hit, usually under 300 yards or so. I believe this is the same study that also showed a vast number of soldiers never fired their weapon in combat that LTC Dave Grossman quotes in his book On Killing too. I am also sure that the Russian's experience in viscous urban combat also compelled their thought process on developing a new round.
Basically, all of the worlds major military powers started to "downsize" their ammo. In the case of the US, really, really downsize it eventually with the 5.56mm cartridge. The Russians eventually followed suit as well and even went a bit smaller with the 5.45 round for the AK-74, but for most the 7.62x39 is still synonymous with the AK. In reality, the .308 (which I will use interchangeably from this point on with 7.62x51 - 3 fewer characters to type) development started after WWI as the US sought out a cartridge much more easily suited for a semi automatic rifle than the .30-06 currently in use. However, the insistence of US brass to keep the .30-06 in use (because we had millions of rounds stockpiled) and the genius of one John C. Garand kept the .308 off the scene for a few decades. In the case of the .308 and 7.62x39, what you have is basically a chopped down full size .30 rifle round. The 5.56 came out of left field and was a round the Air Force was interested in using for base security but eventually found its way onto the front lines of combat for both the Army and Marine Corps. The close in lethality of the round has been proven but its known as being a bit "wimpy" at extended ranges. Basically, the majority of major conflicts in the last half of the 20th century were fought by these 3 rounds.
So when it comes down to it, which one is your favorite? Vote in this months poll, open until Halloween.
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4 hours ago