Any shotgun you have can be used to kill rabbits. They do not have thick skins, so you don't need heavy pellets. You do need a load that patterns well, so you do need to spend time with paper and pattern whatever load you choose to use. That cylinder bore is going to limit the effective range to 25-30 yards depending on what load is used, and what size shot you choose. In a 12 ga. an oz. of #6 shot should produce pretty good groups out to 30 yards, and kill rabbits easily. In fact, #6 shot should work in most shotgun. Even the 1/2 oz. of shot in the .410 will kill a rabbit if you hit it. Since rabbits are so unpredictable in how they run and jump, the more open pattern of a cylinder bore gun is actually an advantage. Getting close to them is generally not a problem. Use 5/8 oz of shot in the .28, 3/4 oz in the 20, 7/8 oz in the 16 and 1 oz in the 12 gauge.
The nice thing about ML shotguns is that you can load them DOWN easily, so if you want 20 gauge performance from a 12 gauge gun, just load the 20 gauge load of powder and shot in your 12 gauge and you have it. You can even shoot a 1/2 oz. of shot to mimic a .410 if you must, out of your 12 gauge.
The benefit of lighter loads in large bore shotguns is that they generally pattern better than the load will do out of the gauge they are designed for, simple because fewer pellets are rubbing against the inside of the barrel in the larger bore.
