A heptibus is a type of long gun with a shorter barrel than a musket. It has seven barrels which can be set to fire individually or all at once. Presumably a flintlock, it is Bunting Faukes' weapon of choice.
The heptibus may be based on the 19th century Nock gun, which also has seven barrels. However unlike the heptibus, all seven barrels of the Nock gun fired simultaneously.