In fact, I would posit that if you test with a longer barrel, you may actually see the opposite effect (even greater acceleration) because the backpressure is higher and the BB spends more time in the zone of acceleration, ie: the barrel.
I think this may be stickied somewhere I remember reading, but just wanted to check. This is true up to a limit, right? Beyond a certain length of barrel, the volume of the barrel gets close to the volume of the cylinder stroke. Then the "suction effect" will reduce fps - the backpressure runs out before the bb has exited. The air rushing down the barrel has expanded to its normal atmospheric volume, so will not expand anymore. Now the exiting bb has to "pull" on the air behind it, creating a lower pressure behind it than the pressure in front. The bb is decelerated again before exiting and fps drops. Presumably there is an optimum barrel length for every cylinder/piston, where speed is at its maximum.