Being honest, the choice if I have to make whilst being realistic would be the AI in 338. Its just that good, fired one a few times, and while it was a bit heavy, and kicked a little harder than a 308, its worth it for its performance.
But, just from pure S&G's sake, from the IWotW, PzB39.
Oh yeah.
Damage and Penetration to dream of, with a range that is in line with other sniper rifles. Not the best I'll grant, but...
