Shot cups with big bores?

That would work at short ranges. It would work better with a smooth bore. "Septicdeath" did a mod on one of his rifles which you might want to search for in the posts. Looked like an interesting idea for something to be used in tight quarters pesting small stuff like rats or pigeons. Rifling is going to be a problem though.
 
The rifling will make the pellets rotate and over distance develop a big gap/void in the middle, like a donut.

I have a 12 gauge with rifled barrel and at @20yds there is a hole in the center of the pattern that will leave a duck unharmed!

Oh and did i say that a normal 20yds pattern from a smoothbore will cover the lid of a 55gal drum, the rifle pattern will cover a man sized target at the same distance.