Now let's look at the other side of the coin - insufficient headspace:
Let's say we have a brand new .308 Winchester rifle with a minimum headspace chamber. It measures at 1.630". We find a deal on some Mil-Surp 7.62x51mm NATO ammo & go to the range. Ammo that averages at 1.6315" will be a very, very tight fit in a 1.630" chambered rifle. Too tight in fact. If you are able to chamber the cartridge you will have higher than normal pressures. Given a few other variables this could result in redlining the chamber pressure & having the rifle self disassemble a few inches from your face. If it's an autoloader then there's a real good chance you'll have a slam fire (where the cartridge fires as the bolt lock up without any desire for it to do so on your part) or an out of battery fire (where the cartridge fires before the bolt is locked) which will cause a self disassembling firearm to materialize in your hands. .0015" is a very big deal when you lack that much space.
Of course the above scenario could happen with .308 Winchester ammo that's larger than average, but it's not as likely as encountering 7.62x51mm NATO ammo that would create insufficient headspace.
So the two cartridges should not be automatically considered interchangeable even though the cartridges themselves are very close to each other. The headspace difference of the rifle's they are chambered for is what makes some combinations dangerous.
