The hand-waving of having "enough" ammo not enough resource management for you? Fine, fine, ammo subsystem.

Some Terminology we'll be using, most of it used incorrectly to real life:

  • Bullet - Misused term for centerfire cartridge consisting of bullet, propellant, and primer. 
  • Shot - An attacks worth of bullets. Varies based on type of gun.
  • Magazine - A collection of shots. Like a quiver for bullets. Fits three to an inventory slot.  

The Quick Way

21 shots worth of ammunition per inventory slot. Done. 

What? Not nitty gritty enough? Alright. 

The Longer Way

Now, internet looking suggests average magazine sizes are as follows:
  • Light pistols - 7 to 16 bullets
  • Modern Semi-Automatic pistols - 15 to 18 bullets
  • Modern Sporting rifles - 20 to 30 bullets
  • Machine guns - 45 to 100 to 200 bullets
Yes, yes, a wide range. Now, given a quiver in the standard system holds 20 arrows in slot, and a bow only shoots one at a time, that gives us 20 shots per slot. That calculation only took 6 minutes using Common Core. Anyway, if'n 3 magazines fit into an inventory slot, and we play it quick and loose with some of those above numbers, that'll give us
  • Light Pistols - 7 shots per magazine 
  • Medium Pistols - 6 shots per magazine
  • Medium Rifle - 6 shots per magazine
  • Heavy Weapons - We've moved into belt fed at this point. 
This gives up 21 shots for Light Pistols and 18 shots for Medium weapons. However, "21" is easier to recall than "21 and 18", so everything gets jammed into 21 shots per inventory. Even Heavy weapons. 

So you wasted all that time to show you're using an extra shot over the core system's standard 20? Yep, showing your work is important and assists in later design processing. Also, the majority of that was already typed out from two weeks ago when I was going in a different direction. Anyway, on to ammo types. 

Ammo Types

Acid - Damage is reduced to 1d4, however damage done is directly applied to the target's armor, lowering the Defense score of their armor by that amount until repaired. 

Armor Piercing - Highly illegal. Ignores 2 points of armor value. 
EDIT: After returning to work on this project a month later, this no longer makes sense to me. If the attack is defeating Defense value, it's assumed to be penetrating the armor. I meant it to lower the armor's Defense value, but in this system that translates to a bonus in Attack. Which is boring. Also, it's being done in a more balanced manner above and below. Sometimes I forget what system I'm in, I admit it.

Depleted Uranium - Leaves traces of radioactive material in the target. Also horrible cancer decades later!

Electric - Target must save or be Stunned for the round. 

Explosive - Does 1 dice size larger (1d4 becomes 1d6) damage, however any critical fails in attacking destroys the gun and damages the user.

Flechette - Fragile projectiles that shatter upon impact. Armored targets are immune to the damage, however unarmored targets receive an additional die worth of damage. (ex: Medium weapons do 2d6 instead of 1d6)

Non-Lethal - Rubber bullets, rock salt, and the like. Does the same amount of damage, however should the target be reduced to zero health with this type of ammo, they fall unconscious, rather than roll on the Death and Dismember table. Obviously, not usable in all weapons, like flamethrowers. 

Paint - Does 0 damage to the target, however a successful attack leaves blotches of paint upon the target/armor until scrubbed off. I...suppose this could be used in a flamethrower...technically. Seems silly.

Phosphorus - Target must save or take additional 1d4 in Fire damage. Flammable objects ignite when shot. 

Tracer - Does 1 die size lower (1d6 becomes 1d4) in damage, however the trace rounds add +1 to hit

Tracker - Not typically used in mass, simply from the cost. Nano computer encased in a projectile that's transmits it's location through out the cloud.