Mods - All three mods. They can be hard to find, but the iron sights work great.
Ammo - Normal .308 will work with sneak attacks. AP when you don't have the luxury of the sneak attack. If you are good at getting head shots hollow points are an option.
Keep your varmint rifle with all three mods handy, and mostly AP ammo, as not every enemy is worthy of a .308 round. A sneak attack with scoped, suppressed, Varmint Rifle useing AP ammo (or head shotting with hollow points) is better than you would think. More so when the "Better Criticals" perk opens up.
10mm SMG, or other such close range weapon, when you are bum rushed.