I believe there is a firm selling 7' or 8' long barrels for domestic shotguns. As I mentioned before, I can't find the link.
Since there is no "device" being used (not even a "choke tube"), these are quite legal (and this is what I thought my link went to )
They use those for aircraft?6 or 7 Feet!! Wow! I think it stops being fun at 28 inches, but that is just me.
Hastings marched the prototype extension barrel through a battery of structural tests, and submitted it to the BATE which concluded it was not a silencer. Hastings then contracted Verney-Carron to produce the unit for the choke tube systems of Remington, Browning, Beretta, Benelli, and Hastings CT11.