For long range accuracy I would suggest a Precision Rifle bullet in a heavier longer bullet, like a 340-350 grn. They will stabilize better at longer ranges.

The only way to really know is to try it. You can't base it on how accurate it is at 50 or 100 yards. If you want to know how it shoots at a longer yardarge will have to shoot it and find out. My Remington with 300 grn XTP's would shoot 1 inch or better groups at 100 yards off a bench. However at 150 yards they were horrible giving me like 4 or 5 inch groups, if that. Had the same problem with my new slug gun and light fields. They shot great at 50 yards but sucked at 100 yards.

If you are going to shoot a muzzle loader or slug gun beyond 100 yards you better check it at those ranges to make sure how accurately it shoots and the amount of bullet drop you get. You should really do that with any weapon honestly.

Paul