I have that barrel and also another invector + ,both in 3" version. I do believe that my rifled barrel is 18.5" and the inv+ is 22.5. You can get an invector choke from lots of mfgrs. as this fits Winchoke, old Ruger, Savage models,etc. The SSX is in invector or inv+, but others such as Sumtoy, Trulock, Carlsons, IC, Kick's and others have chokes to fit this gun. The inv barrel with a cyl bore or imp cyl makes a great slug gun without the rifled choke. Try A Federal Tru Ball in it.
Note: What is set for slugs does not mean that it is fixed for turkey loads to be dead on.