Of course it isn't as hard as aiming a bow. The thing is, even with as easy as it is to aim it's still quite difficult to hit, just because flintlock weapons were that innaccurate. If you take into account the amount of time it took to reload, you're kind of screwed if you miss the first shot. Any Viking who has a bow will know how to use his bow so he won't be as likely to miss. For a Viking, bow usage is a way of life from the time he is very young. For a pirate, flintlock firarm usage is a way of life from the time he decides to become a pirate.