Perhaps a bit of info will help, for certain things long barrels are better, PRB, and real black powder need a reasonable length of barrel,I gethered all the info I could when I was building Hawken rifles and came to the conclusion that more than 35 inches was just extra weight; now if youuse iron sights that extra length will give you the capability of aiming finer.
Now the other side of the story; with a sabot gun you have a choice of weights of bulletsand faster burning powder; the heavier the bullet the more powder you can burn effectively in a shorter barrel length you can use 777 or Pyrodex P fairly effectively in a 24 inch barrel and the most effecent length with a 250 or 300gr bullet is around 26 but a bit more or less wont hurt or help much.
As far as accuracy is concerned it should not make any differance except that for extreme range with heavy loads it will show in the flatness of the trajectory. Lee