without a chronograph youre going to be guessing to some extent anyway. One way you can figure it out is just by shooting groups at different ranges and then interpolating the drop results onto a ballistics chart. Most of the time you will see about 40 fps per inch of barrel, but not knowing what you are starting with makes it a guess. Difference in chamber dimensions and barrels makes it hard to pin down velocities in different guns of the same chambering. I have seen as much as 200 fps difference between guns with the same length barrel using the same load.