[GMFWoodchuck]

Another reason for the handgun bullets is because they are short they do not require as fast of a twist. Which in turns makes your muzzleloader little bit more suitable for the balls and the "purpose built" short muzzleloading bullets like those from Great Plains and who ever else. The twist in your rifle may not stablize a long bullet intended for the 45-70's or 458 mags and etc. But most of the reason is the velocity that a muzzleloader can provide like what txhunter77 said.