OGC:Misalignment (5e Variant Rule)
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Barrel chain rifles and harmonica guns suffer a special kind of misfire. When the bullet is not properly aligned, whether due to forgetting to lock in a barrel or misaligning the slide, then the creature risks damaging the gun and itself. When it rolls a 1 on its attack roll, the barrel is considered cracked (as per the disadvantaged misfire rule) and the creature takes 1d4 damage from powder and shrapnel released by the explosion.