My opinion is that there may be various correct answers as to why straight rifling was used.
1) As Mike discussed, a gun with thin barrel(s) and a fowler front site with no rear sight but having straight rifling was probably for shooting shot with a better pattern. A contemporary maker of swivel breeches told me about 20 years ago that he experimented with straight rifling for shot barrels, and with the right load worked up, he got very good patterns. I have a nice little original French percussion double fowler that fits this bill (both barrels straight micro-grooved rifled). I would not want to shoot round balls from its thin Damascus barrels.
A jaeger wender (swivel breech) in my double rifle study has light barrels, blade front sights, no rear sights, one straight rifled barrel, and one smooth barrel, both 59 cal. I think it was built as a fowler. If straight rifling tightened up the shot pattern, this flintlock would have the equivalent of a cylinder bore and a choked barrel. However, to complicate the issue, the straight rifled barrel should be shot second, is numbered “1”, and is slightly heavier than smooth barrel #”2". Maybe somebody was expecting to fire balls out of it even without sights. After all, some hunters still shoot slugs from shotguns without rifle sights.
2) Dillin reported studies showing that straight rifling grouped better with round balls than smooth bores, but not as well as twist rifling. Those tests should be done well beyond 50 yards in my opinion, since smooth rifles can shoot very well to 50 yards or more, and the advantage of any rifling should be looked for at the greater ranges. (The double rifle that Brent English mentioned in this thread would be a good one for a serious experiment comparing straight and twist rifling, using the most accurate loads that could be worked up. Then add a good smooth rifle to the test as well.)
Straight rifled guns with barrels heavier than thin fowlers, and with rifle sights, may well have been for shooting shot, and also balls with better accuracy than smooth bores.
Many Kentucky double rifles were made with two twist rifled barrels, one twist and the other smooth, and also with one twist and the other straight rifled. They nearly always have rifle sights and barrels big enough to fire balls.
3) The easy loading theory may have merit, but does not explain the many European black powder BREECH loaders with one twist and one straight rifled barrel.
4) Mike Brooks, if you ever find that period documentation on straight rifling tightening shot patterns, I would love to use it in my Kentucky Double Rifle book!
