I built a crank fire for one of my semi auto rifles a video is at
http://s121.photobucket.com/albums/o...ent=jM1919.flv There are also a few still photos in there as well. It’s OK with the ATF but check your state and local Gov’s also.
FWIW the gun talked about above was used in several movies and was for sale in the “Small Arms Review” magazine a few years ago (the cover had a photo of the Grizzly .50). It was listed as having a set of blank barrels and a set of .308 barrels and was listed in the $120,000 price range.