RE: How did they do it?
Your last line nails it, practice. Getting to know your rifle and the sighting system is the key to accuracy along with consistency in all aspects of its use. One thing you may want to consider though is a peep sight. A peep gives you a sense of a scope (round sight picture) while maintaining the non magnified simplicity of an iron sight. Many old Sharps, etc. had long range peep style sights available.