On the straight rifling in your shotgun tube: Such rifling was not meant to shoot ball or slugs in the first place, but to improve shot patterns! Apparently, it was somewhat more popular and longer in use in Austria than in Germany. Some years ago I answered a question on this matter for the German Gun Collectors Association. It was subsequently published in "Der Waffenschmied" #33, Summer 2007. Here is my original answer:
Straight rifling of shotgun barrels is one of those bright ideas in gunmaking which are reinvented about once every generation, just to be forgotten again because the gains are not rewarding the efforts.
The earliest example that fell into my hands was inside a ca. 1760 single barrel, 14 gauge flintlock bird gun (fusee) by one of the Kuchenreutters.
Zimmer: Die Jagd-Feuergewehre, 1877, §15/page 90, wrote that this old idea, which had been already rejected in former times, was in his time again recommended by some unscrupulous gunmakers. He dismisses it as worse than useless! (but he also rejects the idea of boring the barrels with a constriction at the muzzle = choke boring, which was invented then by Pape/Roper/Greener)
I remember reading (about 1970, Gun Digest or Guns&Ammo?) that an American had again invented straight rifling as a vast improvement in trap guns. So I suppose it´s high time for someone to come up with the idea again!
The theory behind the straight rifling is about this:
When the shot/wad column is accelerated down a smooth bore, it is free to get some uncontrolled spin (for instance by the Coriolis force = the force that makes tornadoes twist). This spin would cause centrifugal dispersion of the shot after leaving the muzzle. By guiding the shot/wad this is to be avoided, thus improving patterns.
About 1880 straight rifling was quickly superseded by advanced choke boring, which was much more effective in controlling patterns and easier to make, but could not be combined with rifling (the above mentioned American used interchangeable choke devices with his rifled barrel)
As a bore-fitting round ball could not be used in a choke bore barrel (note the "not for ball" near the proof marks of early choke bore barrels), straight rifling was sometimes retained in ball gun barrels. Ball guns will invariably have rifle type sights! Perhaps it improved shot patterns from these barrels slightly, but it could not do much good to ball shooting: Even if the round lead ball is guided down the bore in a straight line, it is subject to the laws of exterior ballistics after leaving the muzzle, lacking any gyroscopic stabilisation of it´s own. So, straight rifling in ball guns was finally obsoleted by Col. Fosbery´s invention of the Paradox bore, a rifled choke which gives a stabilizing spin to the bullet without disturbing shot patterns too much.
