Over at the ML Forum a guy tried and odd ball load of putting a 1/2" lubed fiber wad over the shot after the normal thin over shot card. Instead of blowing the pattern like one would think it tightened it up.
I tried the same load in my 12ga and got a slightly better pattern, next I tried two 1/8" well greased hard cards and my pattern looked like a swarm of bees at 25 yards. My turkey load is now 90gr of 1F,3 thin over shot cards, 1 1/2" of #6 shot, a thin over shot card and 2, 1/8" hard cards. I get consistent heavy patterns every time.