I have a new project and I want to get some advise.
I’m thinking about making a cape gun, one smooth barrel and one rifled barrels.

It will be a “working gun” so no fancy engraving just a tough shotgun with a rifle barrel sleeved.
I would like a medium range rifle cartridge so no .45-70.
I would like it in .30-40 Krag or .303 Brit. as I can get both .308 and .311 rifled barrels but I do not know any reamer in Argentina for any of them.
I think I can find some used barrels from customized military rifles in .303 Brit or in 8x57mm. The later (I think) can be used to make a 8x57mmR.

Now my questions which cartridge to choose?
Which shotgun? 20, 16 or 12 GA?
Also, any advise on sleeving?

Thanks
Martin

PD: Correct cape gun have the left barrel rifled.