A type of shotgun with two barrels, one on top of the other. That's to say, if you happened to be just in front of an over and under (not the best place in the world to be, if you ask me), this is what you would see:

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Some of these guns have two triggers; others fire the barrels in a fixed sequence - usually the bottom barrel shoots first.
Trap guns will usually have different types of chokes mounted on the two barrels.