The tube sets are $1000-1500 for a set of 3 (20, 28, .410) to fit in a set of 12 gauge barrels (usually fitted to the specific gun). If you’ve dropped $12K on a Kreighoff or Kolar, and had it custom fitted to you, the tubes are a deal.
The idea is that your gun always fits the same in each gauge, since it is the same gun. Almost all top level skeet shooters use them.
Years ago you could purchase full length tubes to turn a shotgun into a “rifle”. You could get them for .45LC and .30-30 and other low pressure shells. I believe they were smooth bore so accuracy was limited to distance. I have not seen them for a number of years. They inserted in break action shotguns and were centered in the bore with “O” rings.