Iv'e owned one of those little Rossi's, in 20 guage, so presume it's the same frame as my 20 guage had a breach large enough to accomidate a 12 guage, and barrels available. Same constriciton where the forend bold was welded on.
My reccomendation is that you pull a slug from a shell and try it in the choke. I believe you'll find it will slip though easily, or maybe just fit snugly. If you open the choke up, the slugs will rattle out the end. And, the constriction will be too small to cause a negative effect on their performance.
So, I suggest you first try shooting it with slugs and see how it shoots as is. If it's accuate, don't fix it. This way you'll still have a useful shotgun.
My Rossie said full choke on the side but was only choked about .006, so I turned the end concentric on the outside with the bore, machined up a choke with I believe about .030 constriction total pressed it on the barrel and tack welded it on opposite sides, just in case, even though it doen't shoot off with 3 inch magnum laods. I then welded steel scope mount rings on so I culd mount a rifle hunting scope. At 30 yards the pattern has only spread a bit over 4 inches, except for the strays which open it to a bit over 5 inches. It is easy to hit running small game and not tear the meat up much by swinging the cross hair just ahead of its nose so the pattern brushes the head. It is sighted for and shoots slugs with very good accuracy, though I've never printed a target on paper with it. Just adjusted to hit on at about 80 yards, and found thtat a 3 to 4 inch target was mine, which is good enough for me, especially with only 2 3/4 inch slugs available in my area.
If you gun is chambered for 3 inch shells, you'll probably not get really good slug performance with 2 3/4 inch shells, due to the long jump to bore diameter.
I haven't explained how to lap the constriction out, because I believe it is a waste of time.
But if you do decide you want to lap it, you'll need to pour a lap skag around a suitable rod, then pull and push it thrugh the constriction till you have it straight. Instructions for the entire procedure are in my lap kit instructions. It is hard work, if you are tough. If you aren't tough you can still do it, by lapping in laps. i.e. Do it till you are tired then again the next day, until the job makes you happy.
