Your Johnny Ring will cast great balls - the ring is pure lead!
I read that folks have hordes or lead acquired from tire shops etc. This lead is good for shooting in smoothbored guns, but very poor for rifles. I have a feeling, when I read that guys have trouble getting a .010" undersized ball down their bores without a mallet, that they are using this alloy of lead rather than pure lead.
As an extreme example, imagine trying to load a ball bearing with your usual patch and lube, in your fouled rifle barrel. If you were able to get it down without breaking your hickory rod, it is likely that your patch is already destroyed and your accuracy will be horrible.
To continue with this extreme example, now load a ball made from something ridiculously soft - plasticsene perhaps. The soft material will flow to conform to the nuances of the rifling, and to allow it to pass the obstruction of the fouling, and the patch with adequate lubrication, will remove all of the fouling, pushing it down onto the charge of powder, and thus leaving a virtually clean bore.
The same thing occurs when using a hard lead alloy, such as wheel weight 'lead', or using pure lead. The finger nail test is ok, but you have to have a test piece of pure lead either in you hand or at least in your experience, to be able to tell the difference. Anyone who has cast a lot of muzzle loading balls from both materials, can tell immediately, just from the appearance of the castings. New cast pure lead balls should shine like a mirror while alloyed balls will have a less shiny appearance - even frosty looking. And balls that have been around for a while will also look different. Prue lead balls will be dark grey, while alloyed balls will still retain their new look much longer. In the bag, pure lead balls oxidize very quickly, turning dark grey almost immediately. Another test is to drop them onto a steel plate. The pure lead has a dull thud while the harder alloyed balls have a metallic clink, or even a ring.
I would have to be desperate to cast balls for my muzzle loaders out of wheel weight lead. But pure lead is not easy to find, and a lot of merchants will tell you they are selling pure lead, when it is definitely not. If you can find lead sheeting of practically any kind, it is almost always pure lead. But be careful - the joint where the lead sheet is joined is soldered with alloyed lead, and must be removed before you melt the sheet, or you will harden your entire pot of lead if you include it.
Good luck.