They're scarce in relation to the common people. But not so scarce for Player Characters.The setting in question, as in your example, is at least as important as the actual firearms rules being used, perhaps more important. It needs to make sense, whatever that looks like for the setting. I'd look at that before the actual rules. In a setting like the one you describe, I'd be fine pumping up the power of firearms based on their scarcity and to highlight why they are restricted the way they are. If, for example, they just did the same d8 as longbows but with crappier range and reloading, there's not much obvious reason to restrict them, narratively speaking. If firearms are rare, and produced mainly for the nobility, I can treating them as master crafted items somehow, perhaps bordering on minor magic items, at least in terms of impact or by comparison to the humble bow.
However, in a fully Black Powder fantasy game, I'd approach it much differently. Guns are common and there's a huge range of quality.
Though having the huge range of quality is a -great- idea, and a way to give players control over their "Magic Items" when it comes to firearms at the very least... After all, guns are recent enough that I'm not going to be dropping them into Dragon Hoards or anything similar. They're not going to be a weapon used -against- the players unless they go up against wealthy folks...
So having better versions or the ability to improve a gun as a downtime activity between adventures could make them significantly more attractive to the nonmagical members of the group who can just pour gold into making them more powerful nonmagically and free up the party's spellcasters to work on other magic weapons.
That is -really- great advice, even if it wasn't intended in that way!