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To unlock the guns, optics and other cadets just play as the class you want to level up and play as it. All the experience you get toward the Assault Class for example, will unlock the other guns (f2000, xm8, AUG, red dot, 4x optics, ammo kit, an-94 akaban, m16)
This also applies to the the other three classes so simply playing the game will unlock everything
After each battle theres a screen that tell the experience you got I'm that round and how much more you need to unlock the next item