I agree with Don on this, you basically couldn't go wrong, stylistically, with a straight barrel. The Huntingdon Co. rifles, made a few miles away, in the same period are mostly straight barrels, usually around 15/16 as Lucky states, and about 38 cal. These guns look slim, but the straight barrel makes them quite heavy and combined with the slim butts they are very muzzle heavy. Given the small calibers I would almost bet that most of these rifles were carried not much farther than the woods out behind the barn to shoot a few squirrels. The butts on many of these guns require you to shoot them off the arm, rather than the shoulder so weight small caliber to give light recoil are important considerations. What barrel I would use on one of these depends on why I was building it. It I wanted a historically accurate copy I would use a straight barrel and maybe cheat down to 7/8"; if I wanted a rifle to carry and use, I would use a swamped barrel following Gary's caveat that it was a recycle. One of my Huntingdon Co. rifles has a 1760-70 vintage, 54 cal. barrel on it with a gradual taper and flare that greatly helps the balance. A barrel I saw at the CLA show and really like in this regard is the Rice 42" J.P. Beck pattern which has a more gradual taper and flare than the B,C, D series barrels.
Tom