Daryl:
You will need a parallel constricted portion of the choke at least 1 1/2 inches long, preferably 2 inches long, in which to cut the rifling.

Good luck finding an existing full-choked shotgun with such a long choke!

...let alone a 'full-and-full' SxS double with 2-inch chokes!

But if you are really really lucky and find one, the easiest way to cut rifling is to set up a small rotary carbide burr on a jig. It won't look 'pukka', but it should work, as that is how the Rem-choke rifled choke tube appears to be made.

empirevr:
The so-called 'jungle gun' had straight rifling down the bore, ending with a twist starting about 5 inches from the muzzles. Yet another variation on the British 'ball and shot' gun.