Out of round can also create problems of course.

You may or may not wish to pursue the special wads that Ed Huble had made up for holing .58 cal. (.575" to .577" bullets. The address for them should be found somewhere in his 12 Gauge from Hell thread.
I have a couple bags of them I bought from a fellow in Manitoba, I think it was. I have not worked with them yet- keeps getting put off, however I do have Blue Dot, Longshot and Steel powders for the experimenting.