From what I've read, back then you were just as likely to die from infection as from the wound itself. The ball itself was filthy, usually greased with lard or whatever, and it would go through clothing thus introducing bits of what the victim was wearing into the wound as well. That doesn't help much though, does it?
One factor in the severity of wounds from muzzle loaders was the amount of powder the gun was loaded with. You could either intentionally or unintentionally "short" the powder load, decreasing the velocity of the projectile. This was sometimes done in duels, sometimes nefariously, to increase the chance of survival. I would think a "short" load hitting a bone would be survivable, barring infection.
And if it's a fictional book, while you want to be as historically accurate as possible, you still have artistic license to work with. While gun enthusiasts go nuts over obvious errors in firearm depictions in literature, I've never heard any critiques on wound survivability (and I'm a member of several firearm forums). Who's to say who could survive what, even today. On rare occasions, you just get lucky.
