The problem is that "normal" powder produces a lot of residue, which does not have any easy way out so it will remain somewhere in your gun until it interferes

The problem is that "normal" powder produces a lot of residue, which does not have any easy way out so it will remain somewhere in your gun until it interferes with any subsequent firing. A musket would need to be thoroughly cleaned after every hunt (or battle), so obviously after a few shots were fired.