Ammunition (or ammo for short) describes the variety of bullets, bolts, slugs, cartridges, batteries, and more that enable all Primary and Secondary weapons to be fired. The only weapons that do not use ammo are Melee weapons.
Remaining ammunition for the currently active weapon is indicated in the bottom right corner as two numbers with a divider between them. On the left side, the number indicates how much ammo is currently loaded inside the weapon's magazine. On the right side of that division, the number indicates how much more of that type of ammo the player is carrying.
Ammo Maximum
There is a maximum amount of ammunition that can be carried for each specific weapon: for example, Warframes can carry 540 units of fuel for the Ignis, but only 20 rockets for the Ogris. Enemies do not track or run out of ammunition, though their weapons do have a magazine size and they must reload accordingly.
Ammo Maximum has no effect on weapons that lack an ammo pool, such as Flux Rifle, Cycron, Plinx, and all Archwing Arch-guns.
Click the "Expand" link to the right for a full list of ammo maximums.
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Magazine Capacity
Weapons have a magazine capacity or clip size that limits their ammo expenditure. Weapons can load ammo up to their magazine size; after shooting the whole magazine, the weapon must be reloaded. Because reloading takes time, a low magazine size can significantly reduce the sustained DPS of a weapon in relation to its expected burst DPS.
Mods that increase magazine capacity
Mods that decrease magazine capacity
Conclave mods
In addition, there are some 
PvP-exclusive mods which affect magazine size:
Ammo Efficiency
Ammo efficiency (or ammo economy) is a metagame measurement of how much damage per shot a weapon can deal. In many cases, ammo efficiency is considered irrelevant because a sufficient amount of ammo packs drop from fallen enemies—but there are a number of weapons with poor ammo efficiency which can easily cause a player to run out of ammo faster than they can replenish their stocks.
Ammo economy is most relevant on weapons with high fire rate. Modding a weapon with damage mods (like 
Serration) increases both damage per second and damage per ammo, while increasing fire rate (through Speed Trigger) only improves damage per second. When playing missions against tough enemies that require many shots to be killed, players should consider eschewing fire rate for straight damage because of this reason.
To compensate for poor ammo economy, all ammo types have a corresponding Aura mod; these are the scavenger mods, which can be equipped to increase the amount of ammo received when picking up an ammo pack.
And lastly, all weapons can equip a mutation mod for their respective ammo type. Ammo mutation mods convert the ammo packs of other weapon types into those for the active weapon, as long as the other ammo pool is full.
Ammo Packs
Picking up an ammo pack will refill only the ammo pool of their respective weapon. However, with the introduction of the ammo mutation mods in the Sling-Stone Event, one can convert different ammo packs as ammunition for the weapon equipped with one of these mods. The conversion will only happen if such modded weapon is equipped (not holstered) and the other weapon ammo is full (e.g. will not convert rifle ammo if your rifle ammunition is not full).
Pistol
- Refills 20 rounds of most Secondary ammo or 1.25 rounds per ammo mutation rank of converted ammo. Certain Secondary weapons, particularly those that deal damage in an area of effect, will only refill 10 rounds.
Pistol Scavenger increases the amount of pistol ammo recovered per drop.
Shotgun
- Refills 10 rounds of any Shotgun ammo or 2.5 rounds per ammo mutation rank of converted ammo.
- Shotgun Scavenger increases the amount of shotgun ammo recovered per drop.
Rifle
- Refills 20 rounds of any Rifle ammo or 1.25 rounds per ammo mutation rank of converted ammo.
- Rifle Scavenger increases the amount of rifle ammo recovered per drop.
Sniper/Bow
- Refills 10 rounds of sniper rifle ammo or Arrows or 2.5 rounds per ammo mutation rank of converted ammo.
- Sniper Scavenger increases the amount of sniper/bow ammo recovered per drop.
- Sniper ammo pickups are the least common drop out of all ammo drops.
Heavy
- Refills all rounds of Arch-gun ammo when used in grounded combat.
- Only dropped by Terra Mankers that appear during Heists.
Omni
- This will refill all ammo of any type of firearm.
- This type of ammo is only available in Conclave missions, Arena missions, and the Simulacrum.
- Not to be confused with the Omni Ammo Box gear.
