I have made 22cal smoothbore zip-guns out of a old car antenna. To sight them you just bend the barrel until they shoot straight.. lol.
To do a proper ‘jug choke’ the over-bored portion of the barrel has to have a section of the barrel near the muzzle bored over the dia. of the muzzle.
It would have to be done in a lathe with a small boring bar.
That would allow the shot to spread out to fill the bore but then be choked back down to less than bore dia.
If I could draw it out I could explain it easier.
I was a tool and die maker in a former life, and I’ve done some work on shotgun choking and reloading for pattern improvement.
Please let me know if you experiment with the concept.
It does not appear that Routledge constricted the bore any, or used a jug choke.
It seems with the small shot sizes, they thought cylinder bore was best.
It appears if you bored and polished a .40 cal hole in a .22 barrel, for 12 inches from the muzzle back, you could have the best of both worlds.
The jug choke concept was used in the Cutts for skeet shooting, and legend has it, the Soviets bought some Winchester Model 12’s with Cutts on them and produced the “Tula” choke.