If you look at guns with choke tubes, you see barrel ends that are swaged out to accommodate the tubes - very ugly and messing with the balance and MOI.
Zippy's pic is close to ideal (the gun make is IMO), although I would also have to include his former partner, Ivo. Then there is the Flodman, Bertuzzi, and a whole plethora of SxS makers
Plastic works fine for soda bottles, not shotguns