When pumping the Tihar (Pellet Gun) or the Hilsing (Spear Gun), you can accidentally discharge ammunition when the animation resets to "fire mode" (because the primary fire button or Right Trigger also works the air pump).

Apparently, the game doesn't tell you the Alternate Fire (or Precision Aim) also works the air lever. If you use the Alt.Fire or Left Trigger to work the air pump, you will not accidentally discharge any ammunition until you are ready (the Tihar and Hilsing only have aim-down-sight for secondary fire).