first ring is a larger thicker ring for the rear of the barrel. chamfer one side to the thickness of the second ring. (see photos)
the second and third rings are the same size but the third is chamfered to the front of the barrel.
paint and fill to make them smooth and space them evenly on the extended barrel (smaller tube inside larger barrel)
then paint the whole lot with black spray paint. i used matt but it can be gloss.