The easy way to do this is to buy a (waterfowl or pigeon) gun with both bbls choked XF, ie 50 to 60 thou, doesn't much matter which maker; get the bores reamed and honed to the same uniform size, exactly (and conveniently a thou or two over the size of the bullets you plan to use), and as close as possible the same choke profile on both sides, but leave them XF. Then send the gun to Ken Owen, have him rifle the choke sections for you. Use circa 20 thou groove depth, so you will have 40 thou engraving by the lands, and another 10-20 thou swaging of the bullet on top of that.

I am not sure of twist but Ken will know. Ratchet is best if you want to use soft lead. You really need to grip those bullets to prevent stripping. And use the slowest twist necessary for the bullet you plan to use (send a bullet to Ken with the gun).

There is another way to do this, for hard lead bullets. Cut the grooves such that the groove dia = the smoothbore dia. ie, only engrave the bullet, don't swage it. With this harder alloy, you can use standard rifling, you don't need ratchet.

And of course you will need to add flip-up sights and have the bbls reregulated (and they should probably be proofed for the load you plan to use plus 20% pressure).

I did a 20 ga this way, using std rifling and no swaging of the bullet. Shoots an NEI Fauneta bullet, only two driving bands. It works great but I have to keep the mv down around 1200, the lead somewhat hard (94:3:3) and the rifling clean of leading. Otherwise it strips and the bullets fly all over kingdom come.