I probably would have said a MRAD in 338 Lapua Magnum, or a M200 from Chey-Tac in .408 but after seeing the 14.9mm SOP rifle and cartridge earlier today I think I would choose that. If you guys haven't seen it google it.
Quote taken from www.thefirearmblog.com
"The 14.9mm SOP is an incredible extra long range rifle project that a number of people at Snipers Hide have been collaborating on. The huge rifle fires a very long and heavy projectile with a high ballistic coefficient giving it extreme accuracy. It has 0.5 MOA accuracy at 3000 yards and remains supersonic out to 5400 yards (5 kilometers). The 14.9mm SOP cartridge case is formed from a 20mm Vulcan case. The 1,690 grain projectile leaves the muzzle at 3,350 fps with 42,104 ft/lbs of energy (that is not a typo!). This is almost twice the muzzle energy of the 14.5×114mm Russian and 14.5mm JDJ. It has more kinetic energy at the muzzle than a 20mm Vulcan, and far more kinetic energy downrange, although the Vulcan is carrying an explosive payload "