I think a 12 bore action would be too large for a 20 bore rifle.
I'm concerned though, that a damascus barreled gun might not have the design or material strength in the action to support
the 20 bore rifle on smokeless, but BP loads suggested should be OK.
A dolls head really doesn't do much to keep the barrels on face as it does nothing to resist hinging forces, only pulling forces.
The 1901 12 bore SxS I used for my 12 bore ball gun, was used with 191gr. 1F & 2F with the .682" ball in a cupped wad. The gun
had .724" bores, tight for 12. I now have a .724" ball mould but the gun is long gone. lol A local guy needed it more than I did.
A note about heavy loads, I did have to repair the stock after the 2nd or third shot with that load.
any modern 20 bore action will handle about any BP load you could get into a case, I'd expect.