For a bolt, partial carrier and barrel I'd explore a remington 74, 7400, 742 ect series rifle. The carrier would have to be modified with a tail extension and something for a piston to push against. I wouldn't make it direct impingment. The barrel already has the lugs in it like an ar15 barrel, and the bolt is already set up for hammer fired. On second thought it might be faster to adapt an 870/1100/11-87 pistol grip stock to a 7400, then mod it for the high cap mags. I've seen 742 demilled guns on gb go for cheap.