Weapon Spawning is a multi-faced feature throughout the game.
This is not to be confused with the Weapon Spawn Hack.
Overview[]
The most common form of weapon spawning (that most players are familiar with) is when a player dies, leaving their "gun" behind. It is not really a true method of spawning, because the player merely drops his-or-her weapon with the same ammo capacity and modifications it had before. There are also some limitations to this, as not every weapon drops upon death (such as Handguns, Specialist Items, Melee weapons, or Explosives).
Another overlooked spawn is the moment when you yourself spawn, with your weapons intact. This quite literally makes you a weapon spawn in your own right. As you may have noticed, in modes that require more than one life, respawning again also reequips you with fully-stocked versions of whatever you originally had, even though you may have exchanged them all out for other weapons that you had found while playing (ex. You start out with a P90, find a M416 CQB, go around killing with it; and when you are killed, you don't spawn with the CQB; you spawn with P90 you originally had).
True Weapon Spawn[]
True weapon spawning only occurs in various Fireteam missions; the most notable being Cabin Fever. In this particular map, weapons literally spawn for players to use at certain points in the game. Not only are these weapons untouched and ready to use, but they also pack more damage than a weapon of their caliber usually would.
As of recently, however, there are a number of hacks that allow players to spawn every weapon that has ever existed in the game; including holiday, limited edition, forged, epic, and even unreleased or removed weaponry. To make matters worse, they can also spawn weapons (that usually can't spawn) such as Specialist or Melee weapons.