Couple of observations:
While I don't know of any citations for boring rifles out smooth, there are a couple of surviving smoothrifles that show traces of rifling. The Brass Barrel Gun, which was bored out too big and burst in consequences, may be one. So it was done.
Smoothrifles are mentioned in period accounts and are known from Germany, though, so it is likely that many were made smooth to begin with. The design of many surviving smoothrifles (octagon to round or round barrels) supports this.
Depends on the gun.
Second, while over the years I have come to agree with Dan Phariss that the average Pre-Rev rifle was probably a bit smaller than surviving rifles would suggest (i.e., around .45-.52, with more outliers above than below though sub-.45 calibers ARE extent), but I'm not sure that the usual explanations of weight and economy really hold water. Light smoothbores of around .60 caliber were and continued to be popular among the Indians in particular, and I don't think that we can chalk that up to greedy traders and ignorant customers. I think any discussion of rifle design has to account for that.
It has occurred to me that period rifles tend to be significantly heavier than the average reproduction, particularly if the thesis that the majority of originals were a bit smaller in the bore than they are today. A six-pound trade gun plus 100 balls at 20 to the pound weighs 11 pounds total, whereas a 10 pound longrifle with a similar ammunition supply at 40 to the pound weighs 12 1/2 pounds. Makes me wonder if the smaller size of the rifle isn't as compensation for the increased weight of the barrel - of course, then we start wondering why they they didn't make lighter rifles...
Finally,
If you want to assess Hanger's remarks on rifles, it might be worth getting to know the man himself. His autobiography (which may not have been published in his lifetime), The Life, Adventures, and Opinions of Col George Hanger is available online. When I skimmed it several years ago I came to the conclusion that he was a conceited snob, and while I still believe that he is a valuable resource I think that his claims to being the best shot in the British Army and seen many hundreds and hundreds of American rifles should be taken with a grain of salt.
Finally,
I think that any multi-hundred-yard shots that may have been made during the Revolution, the ones that the conversation inevitably ends up focusing on when the subject of Kentucky rifle accuracy comes up, are a poor basis for analysis, as are any comparison with 19th century rifles used out west. I am very firmly of the opinion that the 18th century longrifle was never designed as a long range weapon, and those that used them neither normally practiced such shooting nor took such shots save under exceptional circumstances, such as war against a European enemy in somebody's wheat field. Instead, the longrifle was designed for precise shots at fairly close range under low-light conditions - the small caliber, moderate twist, and low sights all indicate use at 125 yards or less. Period shooting matches, I believe, tended to feature single shots at very small targets at ranges less than 100 yards. I also grew up in the Appalachian woodlands, and even accounting for the difference between secondary growth and old growth forests (and how much of the Eastern woodlands was really old growth in 1775? I'm not sure we know) and I can assure y'all that shots over 125 yards are not going to be common.
Those three- and four-hundred yard shots at British officers may make for "rattling good history" but I think they are ultimately a distraction.