The problem with a fully rifled barrel is that it "spins" everything that goes down it - birdshot, buckshot, or slugs. In the latter case, great, that's what it's meant to do. However, with birdshot or buckshot, the force of gravity, acting as centrifugal force, moves the shot to the outside of the "spin". This means that your shot leaves the barrel as a ring around a hollow center, and moves outward (towards the edge of the circle) as well as forward, in the direction of your shot. Thus, before very long, you have an irregular ring of shot surrounding a hollow center with no shot.
You can therefore miss a target with almost all your shot at a relatively short range - I've seen a round of #4 buckshot (27 pellets) put only 5 pellets on target (two in the neck/head region and three "below the belt") on a standard FBI silhouette target at 10 yards. All the rest missed. Another consideration is that all the shot that misses your target is going to end up hitting something (or someone) further downrange. If it's flying out to the side under centrifugal force, its eventual landing-place is uncertain. You might clobber someone well off to the side of where you'd aimed.
So, a rifled barrel is a REALLY bad idea for anything other than slugs.