Have not tried buffer or nickel shot but they may help. In a shotshell loading article I was rereading recently, they talked about lower velocity giving tighter patterns. You might reduce you powder charge to see if the pattern tightens up. The article also indicated slower powders give tighter patterns. 777 would be on the high end of ML powders, and I think Pioneer is on the slow end, with actual BP in the middle. A milder primer may help, too.
