When using an M4A1 or USG45 or .ACP45 pistol and its magazine and the chamber are empty,
Reloading it by clicking Y loads only a single bullet in the weapon's chamber.

While carrying in the inventory (m68 jacket) an extra full magazine and extra bulltes for all mentioned weapons.

The only way to reload the weapons is to:
Drop the empty mag (long Y)
Exit inventory and click Y to reload new mag
Pickup the empty mag