To commemorate me posting over seven hundred and sixty-two times in this forum, let's talk about the 7.62mm cartridge.
Now, as we all know, there are many, many rounds with a bullet diameter of 7.62mm. This also happens to be the measurement of .30 caliber! Without a doubt, the 7.62mm rounds ruled the 20th century, and only in the past 30 or so years has there even been much effort to move away from them in service rifles.
Towards the beginning of the 20th century, we had World War I. During this war the US service rifle was the M1903 Springfield. What did it shoot? Why, .30-06 Springfield ("thirty aught six"), measuring 7.62×63mm.
Here's a picture of .30-06 packed into an en-bloc clip for use in another famous rifle...
... the M1 Garand, the US service rifle in World War II. The Garand featured semi-automatic operation in a battlefield still dominated by bolt-action mechanisms.
The US wasn't the only country using 7.62-diameter bullets, though! The Soviets had the 7.62×54R for their standard-issue rifles. Remember, folks: the "R" stands for "rimmed," not "Russian." That's a common misconception, but now you know!
The 7.62×54R was the Soviet Union's full-size cartridge, used in the ubiquitous and dirt-cheap Mosin Nagant.
It's still in use today by the Russian Federation, most notably in the Dragunov marksman rifles...
...and the PKM medium machine gun.
The Soviets also developed the 7.62×25mm pistol round for use in their WWII-era TT-33 pistols.
There's the boolet, and here's the pistol:
The TT-33 design was widely copied throughout the USSR, with each child nation making their own versions of the same basic pattern.
What discussion of 7.62mm would be complete without the 7.62×39mm round? In the postwar period, the intermediate cartridge began to rule the day, and this round is more or less what ushered it in.
http://www.imperialarmaments.com/image/cache/data/isa76239-500x500.jpgThe 7.62×39mm feeds a couple of obscure guns you probably haven't heard of, like, oh I don't know, the SKS...
...and the AK-47 and its more-prevalent offshoot, the AKM.
And, just for Montrith, the Finnish Rk-62
also uses the 7.62×39mm!
But wait! There's also .30 Carbine!
This round didn't get too much love, but its dimensions are 7.62×33mm, and it was used in the M1 Carbine as an alternative to the Garand for soldiers who needed less weight and more mobility in the course of their duties.
The western nations had their own 7.62mm round, though! Here is the 7.62×51mm NATO cartridge, or simply 7.62 NATO. Its dimensions are identical to that of the civilian-market .308 Winchester round, but very minor differences in headspacing and powder charges mean that the .308 and the 7.62 NATO are not 100% interchangeable, though most modern guns which can shoot one can shoot the other with no problems.
Pretty much all you need to know about this one is that it was used in the FN FAL, legendary among militaries. It goes by the names "the first black rifle" for widespread use of polymer furniture and "the right arm of the free world" for its ubiquity in western-European nations and the countries which received arms shipments from them. The FAL is still in use in many former colonies of Great Britain and France.
The US made their own 7.62 NATO weapon, the M14. While it only had a short stint as a standard-issue rifle until the arrival of the 5.56mm and the M16, it does enjoy a place on the modern battlefield as a designated marksman rifle.
The 7.62 NATO was also used in the M60 light machine gun...
...and its modernized replacement, the M240B, commonly called the "Bravo" in the US military.
As a bonus I will close with an illustration of the dangers of home gunsmithing.