one thing to remember is that in those early days the quality of black powder was variable
a longer barrel allowed a more complete burn and in those days, velocity
Fowlers aside - the longest original flintlock rifle I am aware of had a 54" barrel - add the stock length and the shooter would need to find a tree stump to load from - I actually saw a guy cleaning a reproduction of it - he was standing on a picnic table
please excuse the glare - this duck hunting print was originally done about 1745- this reprint was done when the artist's son joined him in business so third to last quarter of the 1700's - the gun looks to me to be about as long s the hunter is tall. most original fowlers I have handled had barrels in the 42 to 48 inch range