I think this kind of clamp looks more like the one on a M-14 than the clamp-on rails do:
A number of items can be used for the tube (all available at a hardware store): aluminum conduit/tube, hitch pin, etc.
Birchwood Casey makes a number of coating products.
Depends on how realistic you want it to look - more realism requires more expensive materials/processes.
If I ever make one, it'll just be for photographs; I'll remove it for shooting.