For a project I am working on, I had the idea that this new race I wanted to create used a medieval gun as their 'High tech' attack. I figured that someone must have house ruled D&D 0ed/1ed rules for firearms. (Many people have, and more then a few 'firefights' have broken out between the 'lets add guns' and the 'guns are NOT D&D' camps. oy.) Anyway, after reading the boards, I was referred back to Dragon issue #70 where Ed Greenwood took a stab at it for AD&D 1Ed. (He also had addressed canons and other field pieces on an earlier article in issue #60.)
While enlightening, the fact that it was for AD&D made it not an off the shelf solution for OD&D. Also there were some guns mentioned in the Blackmoor supplement, but (spoiler) those were ray guns from amphibious aliens hiding under a swamp. Plus many people have taken their stab at how to come up with the rules- I still need to read the rules that were published in Oubliette Magazine). Most of this focuses on the greater damage guns do, and also the possibility of the gun blowing up in the firer's face. (Something for the player, and something for the DM, if you will.) Many of the solutions also come up with rules to govern range, reload time, and armor penetration.
For 0ed/1ed/S&W/LL I am not too interested in adding additional rules, since the streamlined-ness of the original rule set is one of its attractions. So no additional rules unless absolutely necessary.
Option 1: Pure flavor
The simplest option is to simply take the rules for crossbows and keep them intact, and simple let the player go 'bang' instead of 'twang'. Heavy crossbow is the same as a musket, and a light crossbow is the same as a pistol. Easy, but makes me ask 'why bother?'
Option 2: some new contingencies
Since on the original rules, the damage dice don't change between different weapons, having the bullet do more damage then the crossbow is a little suspect. Anyway, I think the advantage of the gun wasn't its damage over range capability, but instead its armor piercing capability. Which in these rules is governed by the to Hit rules. (since a higher to hit counteracts the defender's AC better) So perhaps a firearm is simply a +1 or +2 version of the crossbow mechanically that is also non-magical.
In addition to that, perhaps we can allow an optional rule where if the player rolls a '1' on their to hit roll, there is a hang fire, with a possibility of it blowing up in the firer's face. (roll a d6 and on a 1 it backfires. on a 2-6 it is simply jammed and can't be fixed while in combat unless the DM decides otherwise.)
As far as rate of fire, I am inclined to think that it is like an encounter power in 4E- that combat is usually too fast and furious to bother with reloading, so you fire once and then switch to the trusty sword or axe. that avoids the whole reload issue. But the rule that it takes one turn to reload, and one turn to aim and fire, might work. Or maybe it needs to be slower. (not sure if reloading & cocking a heavy crossbow is faster or slower then reloading a long arm.)
Option 3: Exploding die
So on the other hand, lets say that we really want the possibility of a headshot with the old gun. Its just cool when you pull it off. A head shot is typically an instant kill mechanic. Some of the solutions I have seen use a hit location table mechanic, (but simple, such as roll a d6 with 1=left leg, 2=right leg, 3=torso, 4=left arm, 5=right arm, 6=head) and then based on which of those you roll, the damage is multiplied based on some rule (limbs are 1x, torso is 2x, and head is 3x your damage roll)
But all of this is a way of saying that sometimes you get bonus damage, since enemies are not homogenous lumps of flesh (except the oozes, gelatinous cubes, shoggoths, etc.) A simpler mechanic is the exploding die, where if you roll your max, you get to roll it again. some rules even let you keep going as long as you keep rolling the max damage on the die.
So by using something like that, the DM can then look at the total damage done, and then describe what happened appropriately. (if someone does enough points of damage to instantly kill a foe, then the DM probably would say you got them right between the eyes.)
Option 4: Divided Rolls
A more math nerd approach then that would be to use a 'Divided Roll'. I came across a very interesting article in Dragon issue #94 about divided die rolls by David G. Weeks called Same Dice, Different Odds which explained this approach. He basically shows that by using two dice where one is divided by the other you can get a asymmetrical probability curve. Depending on the dice you use you can get a very long tail profile. A simple application of this could be where rather then using a hit location table, you simply have the damage roll be a d20/d4. He shows that the average damage of such a roll is 5.6, which is pretty close to that of a regular 1d10. But if the player rolls a 20 on the d20, and a 1 on the d4, then they do 20 points of damage. (20/1=20)
To simulate a d6 damage with a nice log tail for the possible headshot for the gun, a d20/d8 comes pretty close. It averages 3.6 damage, but can go up to 20. That seems like a lot, however 95% of the time it will be a 12 or less, so those 20 point headshots are only about a 1 in 80 occurrence.
Anyway, it is worth it to track down the full article written by David G. Weeks, he does an excellent job of breaking down how the math works. But here is the table of the various combinations of dice he mapped out, that may be of use playing around with divided rolls.