There are some nice firearms rules in the Second Edition AD&D supplement HR4 A Mighty Fortress (see pages 60 and 61).
Here is a summary, for ease of use. Check out the book though, if you have a chance. There are lots of interesting elements contained within.
- Target AC ignores armor (i.e., using newer terminology, is a ranged touch attack); thus, defense against firearms comes from cover and dexterity.
- Exploding damage dice on rolls of 8, 10, and 12 without limit.
- Misfire on a roll of 1 (requires 10 rounds to clean).
- Hanging fire on a roll of 2; this means the shot does not go off, and the fumble range is increased by 1 on subsequent shots until the firearm is cleaned (e.g., after hanging fire once, a misfire happens on a roll of 1 or 2 and hanging fire on 3).
- Fouling (called out as an optional rule): three normal shots count as a hanging shot with regard to misfire and hanging fire chances (to represent the accumulating burned powder.
- Point blank attacks (within 5 feet) inflict +1 burn damage.
- Matchlock firearms: caliver (1d8), arquebus (1d10), musket with rest (1d12).
- Snaplock & flintlock firearms: pistol (1d8), musket (1d12).
- Wheellock pistols: belt pistol (1d8), horse pistol (1d10).
In the past, I have often just treated firearms like crossbows (that are easier to conceal but make more noise). The HR4 rules suggestions seem reasonable, though, and not overly cumbersome.