I have tried the split shot idea and it does work, sort-of. The pellets won't spread out in a ring shaped pattern, the air resistance of the string pulls all pellets together so that they strike in a tight cluster, may as well shoot a slug. Lots of guys shooting cylinder bore muzzleloaders have tried to make paper shot cups to improve the pattern and those often become a slug as well. I've also tried the reverse, buckshot squeezed flat in a vise and loaded in a stack in the shell to increase the spread and that also works, maybe too well. It would be deadly at ten yards but at 20 yards they spread so much they may not hit the target at all. Sort of like the fabled "stack of dimes" load but with lead dimes. I've gotten fairly good accuracy with a single lead ball loaded inside a plastic shotcup, I used a .575" ball in a twenty gauge and a .650" ball in a twelve, about as accurate as a typical slug load from a smoothbore but offering no real advantage over a factory slug load. Ballistic Products offers a variety of odd-ball shotshell loads, well worth checking out their website just for a laugh. It can be fun to experiment with such loads with black powder but with the unpredictable smokeless stuff there is good reason why all reloading manuals caution to use this data exactly as shown and I'd never recommend adding anything to a factory loaded shotshell.