Baikals have always struck me as being quite strongly constructed for a SXS shotgun & for a slug gun, they don't need to handle long brass cases with huge powder charges in the 30,000PSI range - but probably would as several .50/90's etc. have been successfully built on them, I think.

They are innexpensive which meets the criteria requested and have solid ribs, as far as I know, for mounting sights.

Looking over the 'pelters' (pellet shooters) at tradeexcanada, I felt a Baikal, with it's chome lined bores would be the one I'd choose to make up a dedicated smooth slug gun.

If I wanted to rifle the chokes for a Paradox-type gun, I'd go for a double with short 26" tubes, no plating.