The ammo bag is a deployable piece of equipment that the player can use to replenish their ammunition reserves by interacting with it. Like in PAYDAY: The Heist, its rate of depletion is linearly proportional to the percentage of the weapons' ammo pools being restored. The player can carry an extra ammo bag by unlocking the Ammunition Specialist skill, and then gain additional charges by acing the skill.
The Basic ammo bag contains 4 full loads for one weapon. Since every player has two weapons, it will reload two totally empty players to their maximum. Calculation is made in percents, so a full shotgun (30 slugs) is equal to a full assault rifle (90-150 cartridges). It will also allow you to go take ammo when you want without really caring, since you won't use a charge of reloading, but just down the percent. If there is not enough ammo to fill both your weapons, the bag will first fill your secondary (pistol, smg, small shotgun...) and then your primary (main weapon, assault rifle, rifle, shotgun...).