I use the 3" magnum 00 buck
In a 20" Mossberg "Special Purpose", with folding stock and pistol grip, cylinder bore...At 30-40 feet the spread is about 24" which is exactly what I want to see for HD use (since they don't use a shot cup, I think they spread more than normal). HOWEVER, if you use the folding stock, it definitely kills at both ends. Have shredded a refrigerator, and 2 washing machines, as well as a couple dead trees...lets just say they were impressively demolished. Some have said they are dirty, but I haven't noticed any problems.