The "pure lead only for round balls" is a old wive's tale.
Dan is right. I've corresponded with the founder of Cast Performance Bullet Co. He's a rifle builder and used to be a serious competitor. He told me, "If you read the writings of Southerland, Forsyth, Clifton, Cody, and Ashley you'll see ALL of them recommended hardening the balls."
So I recently started casting my own balls from wheel weights. The target below shows they will shoot! I shot this from a rest at 50 yards using my flintlock with open sights. There are 10 shots in the target, though only 9 are distinct. I don't claim to be a marksman and no one has ever mistaken me for one, so I'm pretty tickled.
How hard are the WW balls compared to pure lead? To find out I subjected one of each to the highly scientific 4-inch bench vise squeeze test. Basically, I squeezed the ball as hard as I could with one hand. The results are in the picture below. The WW ball is definitely harder.
I could definitely tell the WW balls were harder during loading. The 0.490" WW balls were much harder to load with my normal pillow ticking patches (0.018"), which I've used for years with 0.495 lead balls. I dropped down to 0.015" cotton patches and the WW balls load easily. That's what I used to shoot the target above.