I'm not sure but I don't think you can drill and tap the barrel or receiver on an 870. That is why they have a Cantilever Barrel option.
You may want to try a B-Square Saddle Mount or their Universal Cantilever Vent Mount. I know the saddle mount works well. I've never seen a vent mount in the wild only on the internet.
http://www.reliablehost.com/bsquare/shotguncant.html
You may want to try a B-Square Saddle Mount or their Universal Cantilever Vent Mount. I know the saddle mount works well. I've never seen a vent mount in the wild only on the internet.
http://www.reliablehost.com/bsquare/shotguncant.html