1) To remove the rear pin on an M16 rifle, allowing the rifle to fold open to reveal the internal parts. This is similar to how a double-barrel shotgun folds open for loading.
2) To explain something in a very simple and clear fashion.
3) To bend a standing woman forward for sex.
2) "Look, let me break it down shotgun-style: the colonel thinks your scenario is shit."
3) "As soon as we finish up this op, you can all go home, grab the little lady and break her down shotgun-style."