Fred,
For your handloads not only add a spreader but cut the petals off the wad. The lead shot will scrape the bore slightly causing each shot pellet that scraps to become a little less round. These will in theory fly toward the outside of a normal pattern.
Another method to negate the venturi effect (
http://en.wikipedia.org/wiki/Venturi_effect ) of choke is to insert a section of drinking straw in the center of the shot column. Close the ends of the straw shut so that no pellets can slip into the area.
The effect of choke is caused by the shot pellets being "squirted" out of the barrel at a higher velocity than the shot moving through the barrel (just like water our of a garden hose). Giving those squeezed pellets an unpopulated area in which to move (inside the section of straw) pellets are not squirted out at a higher velocity.
This is also why the shot string is longer for a full choke than for a skeet choke. Think of water out of a garden hose.
Mark