During a recent visit to an early 19th Century Citadel (Dinant), I learned that the garrison would use special, longer, "fortification rifles". The length of these rifles supposedly made shooting through the embrasures in the thick wall easier. Where there any other differences between these rifles and "normal-length" variants, in construction, accuracy, rate of fire, etc... ?
There's a bit of a blur between what is considered a swivel gun and a wall gun (which is the term I have heard used most often). Swivel guns often had no stock and were instead mounted in forks that were fixed in prepared positions (but they could be pulled up and moved).
Wall guns usually looked like over-sized muskets - often with a calibe between 1-2,5 inches (.100-.250) and a barrel length from 54 inches and up. Some were mounted with bipods, others had a barrel hook to absorb some of the devasting recoil.
Due to the longer barrel, these weapons were usually more accurate than regular muskets.
There are cases of wall guns with several barrels, but otherwise they were mosly the same thing as a musket - only much larger both in calibre and length.