Loose baffle in the end of the shroud ? Or a piece of pellet / crap in there (the last 4 inches at the muzzle).

Can 't think of any other enclosed space that could be capable of retaining an object whilst allowing it to roll around...

removing the shroud requires moderate distributed heat, and some welders gloves. Worst case even if you do damage the finish, it's £20 for a pro reblue on that tube. But shake rattle and roll with the muzzle down first - it should just fall out.