My story is probably newer than most everyone's since I'm still a relatively new shooter, getting more involved in the sport in '09 than earlier in my life. The story starts when I bought a DPMS LR-308 in 2010. To shorten the story, the first box of .308 ammo I got from a gun show seller was marked both .308 Win and 7.62x51. Even then I knew they weren't exactly the same so I bought some commercial .308 ammo and spent some time trying to figure out if I could use the NATO spec ammo in my gun. DPMS marks the gun as .308 Win, and warns against using anything else. Quoth DPMS:
While you can physically fire either 308 or 7.62 NATO from a 308, you will see reduced accuracy from increased wear in the throat of the rifling and a higher chance of erratic cycling, including a higher chance of failures to extract. It is for this reason we recommend using only commercial 308 in a 308 barrel.Pretty serious, huh? If you use Mil ammo in your DPMS .308 rifle, you void the warranty. After cruising forums and various experts, I saved this bit of wisdom.
DPMS does not recommend or warranty the use of 7.62x51 ammo in a .308 chamber.
OK, lets clear up this .308/7.62 business once and for all. Both rounds can be used in either chamber, however there are a few things you need to be aware of if using .308 in a 7.62 chamber.Although I've had that in text file on my computer for a few years, I didn't keep track of where it came from. If it's yours, I'd be happy to credit it to you. For everyone else, heed the last paragraph.
First, realize that .308 and 7.62 have identical external dimensions. Because of this they can both be used interchangeably.
That said, it's important to note that a .308 case has a larger internal dimension than a 7.62 case. The walls on a 7.62 case are thicker than that on the .308. The result is that the 7.62 case is stronger, but the .308 case can have a higher potential load.
This is important to note because for the .308 the combination of a thinner wall, with a higher charge, means the case can be more easily deformed when fired. This is why the .308 chamber is a tighter spec, in order to restrict expansion of the case.
The 7.62 chamber is a slightly higher tolerance (which can aid in reliability, as it is not incredibly picky about round dimensions) which is fine for the thicker walled 7.62 case (thicker case = less likely to deform), but may present an issue when using .308 with a high charge (easier to deform).
Also of note is that while .308 is CAPABLE of a higher charge, it is often not loaded to max spec by manufacturers. 7.62 on the other hand often is loaded closer to it's max spec, and as such they tend to be "hotter" than commercial .308 rounds.
Lets break it down:
7.62 = Thicker case, lower possible max load, but is often loaded to near it's max.
.308= Thinner case, higher possible max load, but is almost always loaded lower than it's max.
7.62 chamber = higher dimensional tolerance (loose)
.308 chamber = lower dimensional tolerance (tight)
You can use 7.62 rounds in a .308 chamber with no problem (cheap surp might be a little tight though), and you can use .308 rounds in a 7.62 chamber as long as you don't reload using too high a charge. If you DO use an overly high charge for a .308 round in a 7.62 chamber, you are likely to deform the case, or tear the head off the case.
If you don't reload at all, you don't need to worry about it (unless you bought handloads).
So sure, ideally it would be good to use the round your chamber was made for, but by no means do you have too. For most, they can use either chamber, either round, and will never have any issue.
And remember people, this is the internet. Do not simply believe everything you read just because it may sound good... even from me (and even though the above is accurate). Please go out and research for yourself in order to confirm what I'm telling you.