Gentlepersons -
As RedCent and Sir C have pointed out: Lower pressure = tighter patterns. larger shot = tighter patterns.
If one is (shall we say) "stuck" with a cylinder bore and is a cheapskate like myself of modest means, one can improve the pattern using the time-honored methods of the muzzle-loading crowd -
One can also tighten patterns by experimenting with buffering media, and shotcups.
A number of folks use scotch tape (on the inside of the cup) to hold the cup petals together for a longer time, achieving tighter patterns.
Many muzzleloaders have created their own paper shotcups utiliizing everything from newsprint, to typewriter paper, to bond, and even the appropriate size paper coin rolls. As always, test carefully, tuning will be required and YMMV, but as the man in the box o' truth says
"shooting is fun"
yhs
prof marvel