i cant argue about the effect on shot, but as far as early and experimental, the straight Vs spiral rifling experiments began with muzzleloaders in the early 16th and possibily the late 15th centuries. the effects where well known but a mathamatician in the early 18th century scientificaly explained the concept. Even gain twist experiments predates pin fire.
so after a couple hundred years of playing around, I would bet this builder knew exactly the result he wanted, it is just that we don't. Dont forget these would predate any cup wads for shot shells so if it is for shot, unless it were wrapped somehow the affect would be limited.
it would be interesting to breach the barrels some how to send some loads (shot and ball) down it to see how they react.
