Bullpup is a distinctive name for a distinctive type of firearm. A Bullpup is a gun with its action placed behind its trigger group, unlike most guns which have a trigger below or behind the action. Because of this unique placement, the rifle itself is generally shorter, and therefore more maneuverable.This weapon layout was likely designed by James Baird in his 1901 patent, Thorneycroft's 1901 carbine. Jack Streat, who shared this great build on moc-pages.com, had this to say about his fully-functioning version of this gun:

"The gun takes a 10 brick magazine (1x4 pieces), any more and the mechanism would jam under the stress of the magazine's rubber band. The bricks are inserted in the magazine in alternating rotations - you put the first one in studs up, then the next one upside-down, then the next upright, and so on. This stops the bricks sticking to each other.

When you pump the shotgun, a rubber band is pulled back, stretching through the barrel, like a crossbow would. when the pump gets near the back, it hits a lever which pushes a brick out of the (inserted) magazine, forward and up to be locked in the barrel, in front of the rubber band. This means that when the trigger is pulled, the bullet is released, and the band can push the brick down the barrel."

Nice work, Jack!