Initial velocity would be zero since the bullet is at rest. That's why I left the inital velocity out of the equation.

If you follow the chart that driftrider posted and the formula you'll see you get pretty close result. With what I entered there was a 138fps drop from the 26" barrel to the 22" barrel. Using the chart 3000-3500fps the velocity should drop 30fps per inch. 4*30=120fps.

Since they are averaging fps loss over a 500fps range I would say that the formula is more accurate.