My 22 Creed is running 82's at 3105 fps. 18" barrel, so I would take away about 100 fps for a 15"
If you do a 15.875 inch length, it would be a tad faster, around 25-30 fps faster than a 15" (Guessing)
Mine groups good at a 1000 yards too.
The 22-250AI or the 22 Creed really shines with the heavy bullets IMO