There are several approximates for barrel length and velocity. The most common is about a 50 fps increase in velocity per inch of barrel length. Again these are approximates and are dependent upon several factors, i.e. bullet weight and type as well as the rate of twist in the different barrels. Were this theory to hold true you'd see an approximate increase of 375 fps for a total of 1675 fps in the 15" barrel. A Chronograph is however the only true way to tell for sure.