[A.Murphy]

Traditionally, you would have to "charge"(Hence the message at the beginning of DoD) your weapon every time you load a new clip.
You do this by pulling the bolt back quickly and letting it fly forward, catching a bullet and making the gun fireable. Modern guns still require charging after a fresh clip is loaded.

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"NO MISSION TOO DIFFICULT,
NO SACRIFICE TOO GREAT,
DUTY FIRST, SIR, DUTY FIRST!"