7.62x39 is .308?

[Geronimo45]

why? why does it have to be so complicated

Let's tell him about 9mm.

A rifle in .308 Winchester or 7.62x51 can't shoot 7.62x39.
.308 and 7.62x51 have some differences in pressures (IIRC), but are more or less identical.
The .303 British is not .303. It's .311-ish.
The .38 Special is not .38. It's .355.
So is the 9mm Luger/Parabellum/NATO.
And the .380.
The .44 Magnum is not .44.
Finally, the Colt .45 (popular name for either the SAA revolver or 1911 pistol) is not Colt 45 (the Billy Dee Williams variety) and neither is .45 Colt (cartridge).

 

[mr_blove]

Not quite, but they're very, very similar, to the point where 7.62x51 can be fired from any .308 chambered rifle, and a tightly-chambered 7.62x51 rifle can safely fire .308 Winchester.

I was told the exact opposite but I gunsmith that I know and trust very well, to paraphrase it he said you could shoot commercial ammo (.223, .308) in a NATO chambered firearm (5.56x45mm, 7.62x51mm) but not the other way around, he said you could have swelling of the case in the chamber, throat errosion, and some other things but I can't remember. It had something to do with NATO caseings having slightly thicker walls and a little bit longer case length, he was saying that you wouldn't notice these differences unless you had a micrometer.