I have bore sighted guns, lots of them, it works great. They have all been with scopes. I have never bore sighted irons before. I just start close and keep making adjustments as I move back until I get he distance I want to be zeroed at. When you have 1 barrel "sighted in" and the other barrel will not shoot to the same POA there is not a whole lot you can do about it. If it is high or low on the shotgun barrel, compared to the rifle barrel - you may be able to use a different load to regulate your point of impact some. For instance in a 20 gauge you could be shooting 7/8 oz loads above the target, but 3" shells will shoot right on. Or vise-versa. Another solution, if you are shooting high or low to a the rifle barrel is to know the settings on the rear sights and move them depending on what barrel you are using. Of course, you will not be able to switch back and forth quickly.