I think I agree with many others that have posted, in that early black powder weapons were costly in themselves, inaccurate and potentially difficult to maintain in a snowy environment. The most frequently stated historical reason for the growth in the use of firearms is the lack of training required to use one, longbows needed daily practice not just to keep up the skill but to keep up the physique required. The other thing that is overlooked, is eyesight, with no vision correction shooting into a mass was the best option for both bowmen and arquebusier/musketeers. I think the as Crossbow is the easy option.
I would suggest that short range and or scatter type weapons would be more use in the confides of what is effectively street fighting. Perhaps a blunderbuss with similar effects to the Scatter Shot spell. For personal weapons which I think should be one shot weapons an increased Fight number in the first round of combat would be the best representation.
One further thing to consider is silver or magic bullets. I can picture a hard pressed wizard reaching for his trusty ‘Demonstopper’, a fearful multi-barrelled one shot weapon as a Demon bears down on him. Previously, loaded with magic bullets and many incantations, the weapon is capable of opening a plannar tear just big enough to send the Demon whence it came if the shot bests the Demons defense. A desperate act but it might not save him and a difficult and expensive thing to come by.