The Ammo Pack is a large, green backpack filled with ammunition. The amount and type of ammo depends on the Ammo Pack. For example, an Ammo Pack found in one portion of a level may have 25 shells, 200 bullets and 10 rockets, while one found later in a level may have 50 shells, 300 bullets, 35 rockets and 10 Sniper Rifle Bullets. The exception of this is Serious Sam Advance, where an Ammo Pack gives the player 100% of every ammo type.
In the games it appears in, the Ammo Pack usually appears before a big battle or during a wave of enemies in a large battle. When it's used in that way, picking up a Ammo Pack usually triggers the next wave of enemies.
Ammo Packs are occasionally found in secrets, usually in the later portion of a game.
If an Ammo Pack does not appear before or during waves of enemies, try to avoid picking up a an Ammo Pack unless one's ammo is low. Picking up an Ammo Pack when the player has a lot of ammo is considered to be a waste.
Ammo Packs found in secrets usually have a lot of ammo in them, so the player should try to seek out secrets later in the game if they're having some ammo problems. An Ammo Pack found in a secret can make a huge difference.