I am wanting to make a cape gun from a 20ga side by side shotgun.

Does anyone make an express sight that flips down out of the way for shooting the smoothbore? (I thought about modifying a 3-blade set by grinding off the first blade and deepening the "v" on the two "flip-up" sights as needed)

What is the biggest bore I can go with for the rifled barrel? I would like to make it a .45 Colt, but an insert maker I have dealt with only goes up to .357 on 20 ga. inserts. I figured that a permanently installed barrel liner would rely on the strength of the shotgun barrel as well as the liner, and could handle a .45, or at least a .38-55.

I figured I should epoxy the liner in place so as not to damage the solder joints and the sights could be dovetailed into the rib. It seems to me that the extractor is the only other component that would need to be modified: am I missing something?

I figured a .45 Colt/20 ga cape gun would be fine for Southeastern hunting and would be fairly light compared to any project on a 12 guage. I started to do this on a 16 ga (with 45-70) but someone beat me to the project gun. This does not seem to be a terribly underpowered combo but I realize that some ideas only sound good, until you hit some issue that had not been thought out.

45LCshooter
If you keep your iron sights low on the rib and the rear v fairly wide they will not be in the way when using the shotgun bbl. I discovered this using my wife's drilling. That way the iron sights are always ready if you jump "rifled game".

I looked at an EAA 45-70 insert for 12ga and the muzzle end measured just at .6". So you might be able to get it to work. Maybe .375 win would be a better option with thicker walls? I made a pair of 470NE inserts for a Belgian 12ga. Right now I have a cracked stock to repair, but hope to have it shooting again soon.

Oh, on the sights, I think Brownells has a folding rear express sight. I got some NECG sights for my project from them. I'm still waiting on the mercury recoil tube though.

Thanks for the input, looks like I have a project.