I just don't want to use anything that may be to hard. Will to hard of an alloy damage the rifling in a roundball barrel?
The lead won't damage the rifling. Getting a ball stuck part-way down and shooting it out without seating it on the powder will damage the rifling, and that might be more likely to happen with hard lead.
The "hard" lead you get from wheelweights will cast larger than soft lead. Being larger, it is harder to load using the same patch--and it won't deform to fit the rifling/take an impression of the patch. And it won't obturate on firing, to fill the rifling.
The one place where the use of hard lead can cause damage to the gun is with revolvers. The hard lead can accelerate wear on the loading lever, or even break it.
For the most part, I stick with soft lead in rifles. I do use a fair amount of wheel weight lead (the "hard" stuff) in smoothbores. My loads were developed with that in mind. A couple of friends use hard lead in big-bore rifles, but again, they developed their loads with that in mind.