It's fun to see something different and this is a nicely crafted rifle. Based on inventories I've seen and surviving examples, its hard to argue that chestnut was used more than occasionally as a stock wood. Regardless of how many 18th century guns survived, the proportion that are found stocked in chestnut should reflect, roughly, the original proportion. And it's always possible that an unusual gun would have an even greater likelihood of surviving.
Period woodworkers were highly aware of the working properties of different hard and softwoods. An example is that furniture items were often made of many dissimilar woods for structural, not aesthetic reasons. Colonial Windsor styled chairs are still being made of 4 different woods.
http://thewindsorchairshop.net/. Hickory, renowned for toughness and resistance to impact as a handle wood, was very rarely used for making handles for guns (stocks), probably because of its instability when wet, as well as its plainness. Most open pored woods are avoided for stock woods because they will wick moisture from end grain, and I believe chestnut is similar to oak in that way.
So to me it makes sense that an abundant wood available in plank form would be used on an emergency basis for stocking muskets, whether or not it met most of the requirements for a gun stock wood: ample hardness, toughness, resistance to splitting, stability with moisture changes, abundance, workability, appropriate weight, and attractiveness.
