Ammo

Ammunition (or ammo for short) describes the variety of bullets, bolts, slugs, cartridges, batteries, and more that enable weapons to deal damage. All primary and secondary weapons require ammunition to shoot, and an equipped primary weapon draws from a separate ammo supply from that which an equipped secondary weapon draws. Melee weapons do not use ammo.

Remaining ammunition for the currently active weapon is indicated in the bottom right corner as two numbers with a divider between then. On the right side of that division, the number indicates how much ammo the player is carrying in total for the active weapon. On the left side, the number indicates how much ammo is currently loaded inside the weapon itself.

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.

 Click the "Expand" link to the right for a full list of ammo maximums. 

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.

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.

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

 * This will refill 20 rounds of any side arm ammo or 1.25 rounds per ammo mutation rank of converted ammo.
 * Pistol Scavenger increases the amount of pistol ammo recovered per drop.

Shotgun

 * This will refill 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

 * This will refill 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

 * This will refill 10 rounds of Sniper 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 pickups seem to be the least common drop out of the current four.