The heavy barrel uses a double feeding system, step one takes the round from the belt, step two loads a round into the chamber (and takes another round out of the belt). Thus to load you have to pull the .50 caliber twice. This is also true of Unloading.
From the Army Field Manual 23-65:
b. Automatic Mode. To load in the automatic mode—
(1) Lock the bolt-latch release down with the bolt- latch release lock.
(2) Jerk the retracting slide handle to the rear and release it. The gun is now half-loaded.
(3) To complete loading, jerk the retracting slide handle to the rear a second time and release it. When the bolt goes forward for the second time, the gun is loaded.
* 3-3. UNLOADING PROCEDURES
To unload the MG, the gunner must first ensure that the weapon is in the single-shot mode. The cover is then lifted and the assistant gunner removes the ammunition belt from the feedway. The bolt is then locked to the rear. If a round is chambered, it will release, unfired, when the bolt locks to the rear. Once the bolt is locked to the rear, the chamber and T-slot are examined to ensure that they are not holding rounds. In darkness, this must be done by feeling the areas. After the examination has been done (during training), a wooden block is inserted in the receiver between the bolt and the rear of the barrel, extending above and below the receiver about one inch. Then a cleaning rod is inserted in the muzzle end of the barrel and pushed through the bore until it can be seen in the receiver. Remove the rod, the gun is now clear (Figure 3-3).
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