I agree with the statement that swamped barrels are by far more common than straight ones on 18th century guns, and also with the point that, on a precarved stock, assuming the job was done right, the difficulty of inletting the swamped barrel is by and large eliminated. Also, when purchasing a kit gun of any quality, the difference in price between the swamped and straight-barrelled options is, in relative terms, fairly insignificant.
My apologies to those for whom the extra hundred or so dollars is, as it very well might be for me, an insurmountable obstacle, and a heartfelt :cursing: to those who seem to believe that, when it comes to this sport/hobby/obsession, money issues can never be permitted to overrule PC considerations.
That said, straight barrels on documented 18th century rifles were not utterly unknown. Rifle #95 in ROCA, attributed to George Schroyer, has a .48 caliber barrel that's 7/8" at the breech; it's possible that it's swamped but far more likely that it's straight. Rifle #50, signed by Jacob Dickert, and also featured in "Kentucky Rifles and Pistols, 1750-1850", is either a .50 or .45 caliber (depending on which book is right) and is described specifically in "Kentucky Rifles, etc" (I don't dare abbreviate that title!) as having a straight octagonal barrel.
If the concept of "commonality" -- the use of only that which was common at the time -- is your ideal, more power to you, and in that case these two rifles don't count. As stated on several occasions before, though, I personally am not going to disqualify indisputably original guns and equipment as possibilities for reproduction, simply because in some sense they don't fit a modern definition of generic correctness.