Bullet Velocity vs Barrel Length
Most shooters have heard that there's a correlation between barrel length and bullet velocity, with the conventional wisdom being that longer barrels provide higher velocities than shorter ones. However, velocity is a complicated subject with many variables including differences in the dimensions of bullet chamber, configuration of the rifling and the type of powder used in the load, and of course the type of bullet used. Barrel length is only one variable to be considered in an accurate measurement, but it's nevertheless an important one. With rifle barrel lengths becoming significantly shorter over the last three decades, from approximately 30" to around 24", it's important for shooters to understand how barrel length affects velocity. It should be noted that at least one respected authority has claimed that the relationship is more or less a myth although our opinion is that its not.
But in general it's accepted that just considering barrel length, type of propellant and weight of bullet, a slower burning propellant is better for heavier bullets but requires a longer barrel to allow the powder to burn completely and provide optimum velocity. However, if the barrel is too long the pressure exhausts itself before the bullet leaves the barrel and the resulting friction between the bullet and the barrel reduces velocity. A faster burning powder can provide higher velocity but at possibly unacceptable pressures. A balance between the three factors is necessary for best performance.
However, let's take a quick look at what's generally accepted about barrel length vs. velocity in a few popular rounds, starting with the most popular round ever, the .22 LR. Since the .22 LR is a low velocity round using a fast burning powder, optimum barrel length is about 16" or slightly more, any longer and velocity suffers.
In the 9MM, this chart shows that though a heavier round than a .22, with a larger propellant load and higher velocity, the optimum barrel length is still about 16" delivering 1600 FPS.
With .45, its bigger, heavier round and relatively low velocity requires a bit longer of a barrel to reach optimal performance, with the best performance found in the 18" to 20" range.
It should be noted that the above refers to these calibers as used in rifles. Performance in handguns is another issue and must be considered by itself.
The barrel length/bullet velocity discussion can be a heated one, as any amount of Internet research will reveal. Shooters should be aware of the many factors that come into play and educate themselves to achieve maximum performance.