Hi, You might be interested in this previous answer I wrote where I take a stab at trying to explain how the power of "muskets" changed over time, as well as This answer discussing musket accuracy.

Yes, there are quite a few military treatises from the 16th century which state the musket as being dangeous to unarmored men and horses at up to 600 yards and the arquebus or caliver at up to around 400 yards. Keep in mind that this had to do with the distance at which the bullet usually still carried enough force to inflict what the authors felt were serious wounds if they hit something, it wasn't really a comment on accuracy.