I would expect that the factory numbers are loosely based on a revolver barrel, length unknown. I wouldn't put a lot of stock in their claims of velocity as they are often optimistic at best. In a single shot you will see an increase over revolver velocities due to the absence of the cylinder gap and longer tube. Bigger bores do not suffer as badly from barrel length loss as smaller bores, the 41 in a 10 would be a nice combo.