Buffering is a wonderful thing, it dates to at least the middle 19th century, of course they used ground bone meal instead of that plastic sawdust we use today. One caution, it does raise pressures a bit so use caution when working with this and Nitro loads, never bed a load that is close to maxiumum pressure.
Another item I picked up that helps patterens is to take a bit of 220 grit wet and dry sand paper and right at the end of the muzzle put this in and turn it enough to put a few spirals at the end of the muzzle, but not enough to remove any metat. The wads will hit this and slow down a fraction, this keeps the wad from hitting the back of the shot string. The shot flies truer because of this.
Shot with higher antiomony will also help, the best shot one can buy is the high antimony shot that is nickel plated, this comes from Italy and can be bought through Ballistic Products. It is a bit expensive for CAS?WAS but is worth every penny in the hunting fields.
One other note, the high antimony shot will also have more of a chance to bounce off of steel targets.