OGC:Chain Fire (5e Variant Rule)
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One problem with early multichambered firearms is that the spark of igniting the gunpowder could accidentally ignite all barrels. If a multi-chambered firearm such as a pepperbox or revolver is used in a ranged attack and a 1 is rolled, then the creature must make a chain misfire saving throw. On a roll of 10 to 20 the weapon misfires as normal. On a roll of 1 to 9 the weapon instead suffers a chain misfire, igniting an additional 1d2 chambers. All bullets fire at the same target, but the additional bullets have disadvantage on the roll and automatically miss if the target is beyond the normal range.
If a chain misfire is used with a revolver, then the gun is also damaged and cannot be used until the gun is repaired with a gun kit during a short rest. If the chain misfire is used with a revolving carbine or rifle, then the creature using it also takes half damage from the extra bullets as they rip through its hand.