As a guide only, you can figure 25 FPS/inch for standard (non-magnum) cartridges. That is, if you cut off 1", you'll lose 25 FPS; add an inch, you'll gain 25 FPS.

However, as Ranger140892 says, it doesn't always work out that way. Whether you lose or gain depends on a lot of factors, one being the case capacity, and another being the burning rate of the propellant used.

For example, generally,a .22 LR HV round attains its highest velocity in 18" of barrel, and anything beyond that slows the bullet down due to barrel friction. This is any extreme example, but illustrates that the same effect will occur eventually with ANY cartridge, if the barrel is long enough. So the question is, "How long does this barrel have to be before it begins to retard the velocity of the bullet?"

The answer, unfortunately, for any particular cartridge and load, can only be determined by shooting trials starting witha very long barrel, from which 1" would be cut after every five shots is fired over the chronograph screens, or some such procedure, (since it is difficult to lengthen the barrel after each series of shots.) You can't "cut it off a little longer".