Like Dan noted, you'll have to shoot it on paper to find out. Persevere and don't become discouraged with many fliers. Most smoothie shooters pick a load and run with that, learning how to hold the gun for whatever range they're shooting. Testing different loads and different patch thicknesses is just as important with a smoothbore as with a rifle.
With 80gr. 2F in a long barreled 28 bore smoothbore, your velocity will be about 1,430fps, depending on patch thickness. The tighter it is, the faster is will be going to about 1,430fps max. That shows about 4" drop at 100 and about 1/2 that at 75yds from a 50 yard zero. Now, being able to see that 4" of drop in the pattern your smoothbore shoots at that range, is the deal here, but that is the actual, given perfect conditions. Shooting a paper target at 100 full yards will be MOST interesting. I've worked with smoothbores what would hold 8" at 100 yards, and others that held around 10"to 12". I'd say that 15" to 20" is most common for guns that haven't been 'worked up' in the load. Guys here know to use thick patches and a ball that gives compression in the grooves - an example would be .620" bore - .595" ball + .020" denim patch. That combo measures .635", with .0075" compression per side when loading. A .54 smoothbore would use about a .515" ball and a .020" patch, giving the same compression and fit. A slightly larger ball might use a thinner patch, but that would not hold as much lube.
What Dan meant about accuracy is correct. Some smoothbore shooters do well out to 100 and even further - I've seen them and they are quite consistant. Our Hefley Creek ML metallic silhouette competition has shown higher scores from 2 of the smoothbore shooters than any of the rifles and it runs to 100 yards in 25 yard increments. It depends on their commitment to finding a load that works well - note that one load may not work well in all seasons - barrel harmonics are different in the winter, spring, summer and fall and many guns shoot different loads depending on 'the weather'. For some reason, thin barreled .54 to 20bore smoothbores seem to shoot quite low in subfreezing weather - just an observation we've made as have the guys shooting them locally. They've learned to "hold more barrel". Guys who try to hold over the target usually end up missing. You've got to see the target to hit it reliably.
Instead of holding the front sight over the target an estimated distance, the most effective shooters learn to raise their eye over the breech, giving the necessary elevation for longer ranges and hold the front blade or bead on the target. This takes practise and commitment. On our trail walk, we have some 90 yard targets and perhaps a full 100 yards on a couple. The longer range targets are usually large enough, the smoothbore shooters hit them sometimes and sometimes hit them often. They are or should be easy with a rifle - if you don't flinch. It's generally the close targets that the smoothies hit easily, that I miss with the rifle -
