Weapons aren't perfect, sometimes they malfunction (also referred as "jamed" in the game). This usually applies only to automatic mechanical weapons with certain feeding system (pistols, SMGs, ARs, MGs, rifles). Most weapons that have splash in the game will never jam, those are plasma weapons and various launchers (rocket, grenades, etc.). If the weapon is jammed, it'll lose all its current ammo and force you to reload it again, plus you will lose a bit of health. There are many factors that can reduce the malfunction rate: perks, blessed or cursed status, etc. Lastly, in Hard Mode, each jam will cause a bigger loss of HP than in the casual mode.
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