Okay, so those of you who have read enough of my posts know that I am a big fan of the Old School Hack rules by Kirin Robinson. Those rules are for an awesomized version of early D&D. The rules skip over boring stuff like keeping track of ammo because elegant simplification is at the heart of the design. But the core concepts in Old School Hack can be adapted to any genre. I've been playing around with some ideas, including modern and sci-fi applications. In modern or future genres you're going to have a lot of firearms, including heavier stuff like grenades, rocket launchers, and heavy machineguns. With modern weapons you're going to decide how to handle ammo.
Now, you could just take the Hollywood approach and ignore ammo. In Hollywood movies nobody runs out of ammo until the plot calls for it. With this approach the DM could call for an ammo check at critical moments. The ammo check (or ammo save) would be a simple attribute check, modified by any appropriate talents, probably using Awareness or Cunning. Characters can get their ammo back by finding or buying more or by spending some APs, maybe 1 AP for weapons which don't fire full auto or a couple grenades, 2 APs for automatic fire weapons, and 3 APs for heavy weapons like rocket launchers or flamethrowers. This approach makes for good dramatic play but is a bit arbitrary and relies on having a DM with good dramatic instincts.
Another approach is to turn low attack rolls into ammo checks. Each time the player rolls the d10s for a modern weapons attack:
- if one die comes up with a 1, then the weapon needs to be reloaded, taking one turn
- if both dice come up with 1s, then the character is Low on Ammo; as long as the character does not fire two turns in a row they can keep on going (the lower rate of fire representing them conserving ammo); but whenever the character fires two turns in a row when Low on Ammo then any subsequent 1 on a die means they are out of ammo for that weapon type.
I was also thinking that ammo can be represented by "loads". Each player normally starts the game with one free weapon; that weapon comes with one free "load" of ammo. When a player is Low on Ammo or completely out they can use up one "load" to clear that condition. Or they can spend APs costing as above. Loads can also be purchased or found.