Why are there dark rings down each barrel of my gun?

Mike George says: If I were you I would take this gun to a gunsmith straight away, because rings can be a sign of barrel bulges.

If the barrels get a clean bill of health, you may as well get the gunsmith to give the bores a good polish. After that, do give the barrels a thorough clean as soon as possible after shooting, first with a bronze brusg and a bore solvent, then with a clean cotton patch or a sheet of kitchen paper, followed by a final wipe with a very lightly oiled patch.

The rings should not re-appear.