7.62 x 54 is a Ruskie round for the Mosan Nagant and
others. I’m sure the poster just slipped.
Indeed. The Russian round also has a rim, while the .308/7.62x51 is a rimless cartridge.
It's my understanding that there *are* differences in the civilian .308 Winchester vs 7.62x51 NATO rounds, mostly in the high end pressure limits of the specification. Use both in commercial firearms, use only NATO in older military guns.
For .223 firearms it's the opposite for different reasons. The chamber throat length specification for .223 Remington is about half that of 5.56x45 NATO, so you can use both rounds in military guns while commercial .223 should use ONLY commercial ammo.
More on topic: I was in line the day after the election to buy a Bushmaster AR15XM, but I donated it to Goodwill out of the goodness of my heart. I no longer own guns. I donated them all to worthy causes because I now understand the right and goodness of Messiah 0bama. ;)
I just bought a Saiga .223 AK rifle and the sales clerk (who seemed very knowledgeable) stated that I could safely use either .223 or 5.56. He pointed out that the extractor on the Saiga is huge and is able to handle both types of ammo. I don’t know if he was full o’ crap but I figure that it’s not really a big deal. I’ll just use .223 if need be.