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 an 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 big difference.