Cart before the horse? Have a barrel blank...

[dverna]

If you build a 15-40 lb rifle, you will not want a break open.

[JimB..]

I’m trying to achieve lowest muzzle pressure possible. Seating the boolet against the powder helps according to quick load. Get a copy and play with it. I think it’s worth the price I paid.

Yes. I’m aware seating deep drastically raises pressure.

I haven’t played with quick load, but I don’t expect that their algorithm fully contemplates what you’re contemplating. Then again, maybe it does.

[Barry54]

I don’t trust the program as much as some people might. I’ve seen it show different pressure and higher charges than published data. I worked down under 1 grain without the program. Seating deeper is what I was missing.
The program shows dramatic pressure changes with deep seated boolets and minuscule charges. I intend to make very small changes and probably at least ten loads for one test sample. I don’t know that I’ll have the courage to go to the limit the program predicts.

[Jedman]

It sounds like to me if you were to find a H&R / NEF Ultra Slug Gun model SBS 980 and have a 24”liner barrel turned to insert in the 12 ga. you would have pretty close to what you want to make.
The USG has a very heavy 24” barrel already and with a 12 ga. Liner it would even be heavier plus you would have 2 calibers 12 ga. Rifled bore plus 9 mm or whatever you want the liner chambered in.
A working extractor would be hard to make for it but you are not looking for a fast repeating rifle anyhow so it could be made simply as a finger nail eject.

Jedman