For a .62 caliber rifle you want a .62 caliber minnie. Check Track of the Wolf and see if they don't have some you could order and try. The Minnie is a slip fit conical. With the hollow base, they are made to expand when fired. This expansion forces the walls of the Minnie Ball into the rifling of the barrel and send the projectile on the way.

I have shot a lot of Minnie Ball and never found them all that accurate. You have to remember, the Minnie Ball was made to kill people. And with the large surface and weight, in most cases all it had to do was hit the person. Bad doctors, and lack of modern medicines did the rest for the wounded.

Instead of a Minnie I would be looking at a .610 roundball. Shoot them starting out with 60 grains of 2f black powder or Pyrodex. Let the weight of the bullet do the work for you.