The Citadel of Shadows
The GM picks a type of die (d12, d10, d8, etc…) based on several factors, which we’ll explore below. This represents full ammo for that scenario–so a player might record on her character sheet: “Ammo: d12.”
When a player rolls a “1″ while using a firearm, the die type decreases by 1 type. So, a d12 becomes a d10, a d10 becomes a d8, et cetera, down to the lowly d4.
When a player rolls a “1″ on a d4, she has one arrow left.
At the end of a combat when ammo has been used, the player drops to the next die type. In the example given, if the player began with a d12 and never rolled a 1, she would use a d10 at the next combat. If the character is able to find a stash of ammo through the course of adventuring, she may increase a die size. And if a character is able to “get back to town” (that is, get a reliable supply of more ammo), she may immediately jump back up to an appropriate carrying limit as the GM decides.
Adding color to the crunch
Abstractly, this system represents a character possessing a relative level of a consumable. For instance, using a d12 with ammo represents an average of 40 arrows, but it could be as few as 5 (for the unlucky bastard who rolls all ones: 0.0043% chance when starting with a d12!), or more than 100 (in 1,024 rolls starting with a d12, only 5 were lucky enough to fall in this category).
But what does it actually represent?
To illustrate, let’s go back to the 1e “combat round” that took a full minute. The idea was that the attack roll represented the success of a minute’s worth of effort–this could be one telling sword swing, but probably was a dozen exchanges before a good meaty thunk.
Why not do the same with arrows? Some rounds, you might get one arrow off, and others might be a barrage of 5 or 6. Other times, in the heat of combat, you’re just not keeping a very close watch on your arrows, but in a moment, you’re surprised by the lightness of your quiver. These situations are what happens when you roll a “1″ and go down to the next die size.
When you’re down to a d4, you’re going to be a lot more careful with ammo than you were when you used a d12.
What die to use and when
To get an average number of shots, simply add the values of the die. Using a d12 (12+10+8+6+4) gives an average of 40 shots, whereas using a d8 (8+6+4) gives 18 shots.
The d12 feels like a good starting number for GMs who allow their characters to carry 40-ish arrows. If you’re more of a 20 arrow kind of person, go down to an 8 (18 shots) or 10 (28 shots). In a “defend the castle” situation, where there are plenty of arms available, you might even use a d20 to start with.
* Want to share ammo? No problem: “crack” the die. A player may voluntarily drop a die size to increase an ally by a die size (for instance, Rowan has d10 arrows left, and Johanna is down to d4. Rowan voluntarily drops to d8 in order to increase Johanna to d6.)
* Don’t just use this for ammo–it works for food, too! Characters can roll once per day–a 1 represents either that they’ve eaten a lot, or that some food has spoiled, etc… Successful hunting lets characters skip rolling for a day, and something like the “purify food and drink” spell might allow characters to increase the die (go from a d6 to a d8, for instance).
* This works well for arrows, but is a bit harder for guns that have a limited number of shots before a reload. If you worry about reloading in your adventure, my suggestion is to keep track of your clips and use this system for bullets in the gun (using a d6 or d8)