200 yards with open sights will take a lot of practice, no matter what rifle you get. The .54 caliber might be a better choice but as Lonewolf said, they are going to be hard to find. With a sabot or big conical the .50 caliber will do the job.

Seeing your from Colorado I am guessing that sabots are out of the question also. For this reason you are going to want a quality long range shooting concial rifle.

The first and best one that comes to mind is a White Rifle. The Super 91 or some of the new ones in .504 caliber will throw that 460 grain No Excuses concial accurate to as far as you can hold it. White Rifles and concial seem to have a real love affair.

Some other options would be a custom type rifle. My Renegade with a .58 caliber GREEN MOUNTIAN barrel is accurate to 100 yards but with open sights I would not try a 200 yard shot. They do make Green Mountain barrels in faster twist and .54 caliber. Whether it is a conical shooter, who knows.