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TL;DR: If you pick up a dropped weapon it will always have full ammo in the clip.

1)Deplete the gun's current clip (0 ammo is fine, but obviously works with whatever above 0 too)
2)Drop the gun (CTRL+D)
3)Press the grab all key (G)
4)Cycle your weapons back to the original weapon (Q)

Doesn't work with energy weapons, but who needs that when you can just spam RP-32, GL-303, AR-7?

Those poor guards, getting mowed down by AC-88 in the 1st mission...