Check the length of all 7.62x51mm cartridges before using them in a .308 Winchester chamber.

If you're read all the way through the admonitions about not using .308 Winchester in a 7.62x51mm chamber, then you know that 7.62 chambers are dangerously long compared to what's allowed for .308 Winchester. Interestingly, the cartridge dimensions themselves for 7.62x51mm are identical to .308 Winchester. All that extra space is just to improve reliability of feeding and prevent headspace issues in firearms that love to slam the case into the chamber with enormous force; e.g. machineguns.

The reason I check the case length of military surplus ammo is because that extra headspace is forgiving to cartridges that might be slightly oversized. Ammo that might never have an issue for a military using 7.62x51mm arms could prevent a bolt from going into battery on a .308 chamber. In a bolt-action rifle, this could cause headspace problems or overpressure if the bolt is forced closed. In a semi-auto, it could cause a slamfire.

I've yet to find a 7.62x51mm cartridge that fails to fit a .308 case length gauge, and I've checked every single one that crosses my path. However, there are anecdotal accounts on the web that over-long rounds have been encountered. Case length gauges are inexpensive and can be used to check hundreds of rounds while watching a movie. Given that it only takes one out-of-spec case to destroy a gun and/or parts of the shooter's body, it seems like a reasonable investment to me.

You can buy a case length headspace gauge here.