This will not apply the same to light loads as it does for me as I like a load in the gun when I shoot it.
With PRB the differance between a 22" barrel and a 32 " barrel with 120 gr RS was 400 FPS
With inlines it changes a lot in all directions the amount of powder the weight of the bullet enter into it.
For instance with a 500gr bullet in a sabot in a 54 cal you can burn the 150 gr load of RS quite well in a 22 inch barrel. but with a 200gr SW in a 50cal in a 28 inch barrel you find about 10 or 15 gr of unburned powder if you streach a white sheet out length ways 5 feet in frount of the barrel on the ground.
Its real hard to answer your question with out more info.
The most used bullet would proably be a 250 gr put in a nice tight sabot for good accuracy with the most commenly used powder charge of a 100 gr the 26 inch barrel wold do just as well add 25 gr and the 29 inch barrel would start showing an advantage in FPS. Lee