The shallower the rifling the better(to an extent). Shallower rifling yields less bullet deformation. Thus better acuracy from a less deformed bullets.

Optimum twists are between 1in18 to 1in36 for bullets. But as I said earlier the slower twists will work also. Just experiment.

As far as casting an MZ, there are several ways. I normally just do it with my lead ladle that I use to pour bullets.

First make sure the bore is oiled well. Then push several patches an inch or so into the riflings with a 16D nail. 20D nails are better. Push the patches down the shank of the nail until they rest on the back side of the head of the nail. Then push these patches down the barrel. Once you are satisfied that there is a good obstruction of the barrel then pour the lead down the barrel(about an inch or slightly less thick). Then let it set up. Take a pair of pliers and pull the nail. You can also do that, and I recommend doing that to the breech end also. If you have access to the breach end. Just use a longer nail. And make sure you have the barrel oiled well. No problems.

It is best to use cerrosafe to measue your bore. but I have done several with pure lead. You can buy a piece of Cerrosafe from most supply houses. It is reusable. Tom.