1. Approach the kneeling marksman holding the Mk18 ABR (http://screencast.com/t/nZeO0qLzr)in the Gori hub area and take the 7.62mm 20Rnd Mags from his pack.
2. Take all 7.62mm 20Rnd Mags from all boxes in the armory area.
3. Go out on patrol.
4. Immediately return from patrol.
5. Return to the marksman and note that 3x 7.62mm 20Rnd Mags have magically appeared in his backpack.
6. Repeat as necessary and never fire anything but the Mk18 again.

*Note: If you skip step 2, the marksman will simply take his mags back from the armory once you leave. It's also worth noting that you cannot store mags in non-armory boxes in the Gori hub(such as the box the rifleman facing our marksman is sitting on), as their contents will be consolidated into their proper armory boxes upon triggering a loading screen.

I love my Mk18 ABR, and I never leave any stone unturned when searching for ammo to feed it. The relative scarcity of 7.62 NATO ammunition meant that when I raided an enemy encampment and found 10 Mags stashed in an ammo box, it was cause for celebration. This duplication exploit totally kills the joy of raiding an enemy checkpoint in the hope of just ONE MORE BULLET--a vital part of the immersion when fighting in an under-supplied guerrilla army.