RE: accuracy problems
I buy my pure lead for casting from Midway USA. I also got a batch just last month from a plumbing supply house. If the bullet design you are using is not working then you need to find one that will. You said you have tried different powder charges. The minnies are a slip fit projectile made to shoot well from a dirty barrel. Do you get better accuracy as the rifle gets fowled?
If your minnies are real loose fit (as mine are) here is what I did. I took some white teflon tape and wrapped the minnies with a couple layers. I then lubed the tape and shot them that way. The tape increased the diameter of the minnie and made them engage the rifling better and hence the accuracy got much better.
Otherwise get some different .58 caliber conicals (not minnies) and try them. I think Hornady makes some.