I don't know how long crossbows 500 years ago took to reload but based on wikipedia and movies I've seen (great reliable historical reference's, I know ) a civil war soldier was expected to get up to 3 decently aimed shots per minute. Now it probably would take two or three times longer to loose that many bolts with a crossbow in the 15th century but the bows Mat has can fire 6 or 7 bolts per minute. The one thing muskets definitely have over bows though is the physiological effect, seeing all that smoke and hearing those loud bangs and bullets whizzing by your head will definitely make you s*** your trousers.

Now maybe we will see guns in the series. But I just though based on Mats conversation on how he was looking for something that could fire even faster and that he asked Aludra who says she's not very mechanically inclined that something like the Hwacha might me a more likely possibility. The other possibility is a hand cranked gatting gun like those used during the American Civil war and just after, but that is a bit more complicated to make and would involve Aludra having to invent bullets with rifling and shell casings.