How i did turn my ANH Hero MGC down (should be even easier with a Denix, as i don´t trust the MGC barrels, solid, hollow, with hard steelrod inside - found to many versions, Denix barrels are always massive).
Upper receiver disassembled, barrel tip in the chuck and in the rear bolt-hole secured with a revolving center in the tailstock. Not to fast and not to deep passes, very sharp tool.
http://i527.photobucket.com/albums/cc352/Lichtbote/Props/Blaster/MGC-ANH-Hero/anh-bullbarrel001.jpg
Don´t part it off in this position, saw the small post.
http://i527.photobucket.com/albums/cc352/Lichtbote/Props/Blaster/MGC-ANH-Hero/anh-bullbarrel013.jpg
Way easier if you just need a little bit smaller diameter:
http://i527.photobucket.com/albums/cc352/Lichtbote/Props/Blaster/MGC-ANH-Hero/guns004.jpg
Something like that can be done without lathe, just a good sized workbench drillpress and a sharp file is needed. Again tip in the chuck, and a center in the vise. Precise adjusting that there is no wobble, slowest speed and carefully filing.