Barrel Length vs. Muzzle Velocity
Lots of manufacturers list the muzzle velocity (and other ballistic data) for their ammo. And some of them specify the length of the test barrel used to develop the data (for example Federal uses a 24" test barrel for many of its hunting loads).
My question is this:
Is there reliable rule of thumb that will tell me the reduction or loss of muzzle velocity if I use a shorter barrel than the manufacturer's test barrel (say, for example, my barrel is 18" or 20" long and the ammo manufacturer tests with a 24" barrel)??
p.s. I know the best way would be for me to use a chronograph and test the ammo, but I don't have one.