Frankly, you could just use crossbow stats for an early matchlock arquebus -- it's not like the combat system is realistic to start with.
The
Strategy Page's
Infantry Missile Weapons in the Renaissance covers the topic in some detail:
By 1500 infantrymen had three different missile weapons available to them. There was the arquebus, a relatively light firearm manageable by one man, as well as the very common crossbow, and the longbow, which was mostly limited to use by the English. Technically the arquebus was inferior to both the other two weapons in range, accuracy, and rate of fire, while the longbow was generally superior to the crossbow.
[...]
The inferiority of the arquebus to the other two weapons was actually even greater than the data suggest. Since it was subject to fouling due to the build up of unburnt powder in the barrel, the effective range of the arquebus tended to decay after a few rounds. So it would certainly be reasonable to conclude that the arquebus was in every way inferior to the two older weapons. Technically, this was precisely the case.
But the arquebus possessed several advantages over its two rivals.
Relatively speaking the arquebus was cheaper than either the longbow, which had to be meticulously handcrafted from yew, and the crossbow, which required equally meticulous workmanship and rather expensive steel as well. The arquebus could be mass-produced by a foundry in fairly cheap cast iron. In addition, while the range, accuracy, and effectiveness of an arquebus round were inferior to those of the other weapons, an arquebusier could carry more ammunition than either of his competitors. Arquebus ammo weighed less than arrows or crossbow bolts, even after adding in the powder charge.
[...]
The arquebus had one more very important advantage over its rivals. It was perhaps the critical advantage in determining the rather rapid conversion of armies from archers to arquebusiers. A man required considerably less skill to become an arquebusier than either a crossbowman or a longbowman. A few weeks training was all that was necessary to turn out a fairly capable arquebusier. In contrast, it took years to properly train a the bowman, who had to develop considerable musculature before being able to use his weapon to its fullest capacity. This was particularly true of longbowmen, of whom there was a saying that in order to a good one you had to start with his grandfather.
Naturally, D&D's firearms rules get every one of those elements wrong. They make firearms harder to use (exotic weapon proficiency vs martial or simple), more expensive (500 gp vs 75 gp or 50 gp), with no lighter ammo (2 lbs vs 3 lbs or 1 lb for 10 "rounds"), but more damage (1d12 vs 1d8 or 1d10). And they give firearms and crossbows ludicrous rates of fire (five rounds per minute).