I've read that a 1:60 twist the longest conical that could be stabalized would be .50 approx. ( http://home.insightbb.com/~bspen/ in "A little Math" )
In a .50 cal well a ball is about it. I'm going to get some buffalo ballets and some Keith FN from PR Bullets and try them. both are about 240 - 300 gr in weight.