It would be more interesting to use your skills for good not evil AKA make a profit.
Since the gun you are wanting ot make would run 20lbs fully loaded, not great. Stay away from bottom ejection like Ithaca. No gun jams more and when that type of action jams, it is major cluster.
A 15 to 20lb gun with two actions to work would be SLOW and cumbersome compared to any shotgun but especially a Saiga with a 20 round drum.
Look at the Neostead. Make that in America. Good niche gun, not sure if it is patent protected etc............the answer to your quest lies in there some where. South Africans liike shotguns Ask Awerbuck.
