[@Sophrus]Fallout 3's sniper rifles are chambered to fire .308 rounds, not the .50 BMG you mentioned in your post.
[img]https://upload.wikimedia.org/wikipedia/commons/a/ab/Rifle_cartridge_comparison.jpg[/img]
.50 BMG is the first on the left, .308 is third from the left. There is quite the difference.