Strongbox

Strongboxes are items that appear in. They are the method of obtaining Weapons and Armor, rather than buying weapons in previous games. You can find strongboxes in various locations throughout the game, but they are primarily located in secret rooms and can be obtained from killing bosses. Multiple bosses do not give multiple Strongboxes, and the amount of Strongboxes obtained from a boss at the end of a mission is limited to one. Additionally, objects like vases in the VIP Mission can contain loot drops, including strongboxes. Killing zombies also has a small chance of dropping Strongboxes, and collecting a Daily Reward may also award Strongboxes.

Tiers of Strongboxes
There are eight tiers of Strongboxes you can find. In order from lowest to highest tier, they are:
 * 1) Steel
 * 2) Titanium
 * 3) Molybdenum
 * 4) Iridium
 * 5) Neodymium
 * 6) Promethium
 * 7) Thulium
 * 8) Nantonium

Properties

 * Each Strongbox opened contains exactly one weapon or armor piece.
 * The higher level the strongbox, the better items you'll get. That does not mean that lower-tier Strongboxes will always give undesirable items, and that does not mean that higher-tier Strongboxes will always contain something desirable to the player.
 * The higher level the strongbox, more "flash" you'll see from collecting the box in a mission. The max flash has lightning bolts fly from the center of the screen in random directions and the screen turns white twice in quick succession, while the lowest level has nothing.
 * When looting a relatively superb gear from the strongbox, there will be bright flashes and a glowing text saying "High Level Gear". It only happens when the gear is relatively average or better AND with good aug max level of over 9.

Trivia

 * The Steel and possibly Promethium, Thulium and Nantonium strongboxes are the only ones colored like the material they are made from is.
 * Titanium strongboxes are green when it is actually silver-colored, Molybdenum strongboxes are blue when it is really gray, Iridium strongboxes are blue when it's actually silver-colored, Neodymium and Promethium strongboxes are purple when Neodymium is actually silver and Promethium's color is a silvery-white, though in some forms, it's a blue-green glow, and Nantonium strongboxes are bright yellow, but its true color is unknown due to Nantonium being fictional.
 * There is, however, a possibility that Nantonium is naturally silver-white. A NK game called Magnetic Defense features a special item called "Nantonium Ball". It is a large, silver-white sphere. Apparently, Nantonium is very resilient, as the Nantonium Ball on Magnetic Defense can easily plow right through long lines of tanks.
 * Neodymium and Molybdenum are the only strongboxes to have their own special light color, as Thulium and Iridium share a greenish light, Nantonium and titanium share yellow, and Promethium and Steel share white, with Neodymium having purple lights and Molybdenum has light blue lights.
 * The mobile Version for SAS zombie Assualt 4 does not have the same graphics for the strongboxes instead they look more saturated in color and only have lights seen from the sides not on top.