Barrett Model 99 in 0.416 caliber, although I wouldn't turn down the .50 cal. I sold a .50 caliber to a customer last year who shot a deer at 900 yards on his brother's farm... I won't post the details, but he didn't have to clean it. He hit it on the first shot. He made a standing offer to anyone at the gun store to go shoot the BMG, just have to buy the ammo (at $4.50 a shot!).
On Military Channel -
Future Weapons:
In a second season episode of Future Weapons the host (Richard Machowicz, a former Navy SEAL) engages in a shooting competition with another sniper. "Mack" achieves a cold-bore first-shot "kill" at 2,500 yards (2,286 m) using a .416 Barrett Model 99 rifle while his competition, using a .50 BMG, required 3 shots to achieve a "kill".
Now, THAT'S a real weapon!