Part of an accurizing program on a levergun is to loosen the mag tube so that point of impact don't change when the barrel heats up. Perhaps a gunsmith versed in leverguns can help you out. The degree of looseness may be too much, or it may be beneficial.
The magazine tube is screwed to the underside of the barrel near the end of the tube closest to the muzzle and shouldn't be loose due to the barrel band. The barrel band's main function is to hold the forend in place, and like Ruger No. 3 said, a certain amount of looseness there is beneficial. If your mag tube is truly loose take a look at the screw that passes through the tube at the end and make sure it is tight.
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