It would work as a bag for a fowling piece, probably not exactly accurate even to the "late"er part of our period. It would help if you took a seam ripper and remove the zipper, and find a way to change out the strap buckle on the strap holding the flap closed to a more simple one without the roller.
It is a very nice, seemingly well constructed bag. It will looks like it will hold more than you will ever need in the field.