Staring at them in the eyes, you can challenge any other player to a duel, regardless of distance.
The challenged player may discard a BANG! card (even though it is not his turn!). If he does, you may discard a BANG! card, and so on: the first player failing to discard a BANG! card loses one life point, and the duel is over.
You cannot play Missed! or use the Barrel during a duel.
The Duel is not a BANG! card. BANG! cards discarded during a Duel are not accounted towards the “one BANG! card” limitation.