*UPDATE: Vash released a master ammo file along with his MK23 and TRG weapons which has a .50AE, .45ACP, .338 Lapua and the 7.62NATO ammo in it. The scripting of the 7.62 ammo in the new master file is compatible with the original FN FAL file so you do not need to re-download anything and no updates are necessary.*

This is a quick tweak mod I made by request of Kjohns339.
This is the first mod I ever released, took me 10 minutes to make but I did test it. I Also made a version which uses the 7.62 NATO bullet which was made by Vash. You need this FN FAL mod for the 7.62 version to work right:
http://www.fallout3nexus.com/downloads/file.php?id=569
Alternatively you can use Vash's H&K MK23 mod or TRG mod which both contain a master ammo file which includes the 7.62 bullet. I also highly recommend them along with the FAL mod as they are both excellent mods.
H&K MK23
http://www.fallout3nexus.com/downloads/file.php?id=640
TRG
http://www.fallout3nexus.com/downloads/file.php?id=931
But if you have something against the FN FAL, TRG or HK USP the HK and TRG mods have the master ammo file separate from the weapon file, so you don't need the sandwich to have the cheese (although the sandwich makes the cheese way more enjoyable).
So there are a lot of ways of getting the ammo you need for the 762 version, even though it doesn't exist in vanilla F3.

The 7.62 and .308 versions are otherwise identical.

Changes the assault rifle from
Vanilla | Modded
Bullet:
.223 | .308
Damage:
8 | 20
ROF:
8(3 rounds/burst) | 6(2 rounds/burst)
AP usage (per burst):
23 | 30
Accuracy:
low | high
Mag Cap:
24 | 20

Everything else remains the same. (looks, price, weight, animations, sounds)