While browsing auctions at GunBroker I noticed a fascinating Chinese fortress rifle from the late 1800s. The rifle is 7 feet in length, about height of a modern doorway, and weights 40 pounds! A gun of this size would have been used to snipe from the ramparts of fortresses.
The rifle is about .75 in caliber (about the same caliber as a 10 gauge shotgun). Judging from the photos, I think it fired mini-artillery shells about 6.5″ in length (about an inch longer than the modern 14.5×114mm Russian).
It must have been designed by a very careful engineer as it has 16 rear locking lugs on the bolt. The problem with adding a lot of lugs is that it requires precision metalwork to make them all engage the action (or barrel extension) at the same time.
The rifle had a buy-now price of $3,600 and did not sell.