Is the gun grouping well? If not, I'd suspect the mounting system. If so, try switching the bases around, front to back.

If that doesn't cure it, if you're shooting slugs that are around 2000 fps, try a slower variety.

If there's still a problem, could be the scope, but also bent barrel.

If the gun has iron sights, will it sight in at 25 yds without jacking the rear sight up an unusual amount? If so, it's probably the scope, if not, then maybe it's the scope.