FYI - In firearms, barrel length does not actually affect accuracy in and of itself. It is indeed a widely held myth though. A longer firearm barrel can/will increase velocity, which at a distance will decrease range drop, and wind deflection will have a reduced effect on accuracy. Also, a longer "rifled" barrel will allow for a more stable spin on a bullet helping with accuracy. Two bullets exiting two identical smooth bore barrels at the same FPS, with one barrel being 1' and the other 5', will have the same accuracy. Make those barrels rifled, and the accuracy will be greater with the 5'. Or, using a large cartridge with plenty of gunpower, the the 5' will be more accurate because the bullet will have a higher velocity.
My current theory is that a 5' blowgun has the same accuracy as a 3', assuming that the darts are leaving at the same speed. I have this theory because (I believe) the accuracy of the dart is more driven by the aerodynamics of the dart (cone shape), once it leaves the barrel, than the accelerated mass of the dart itself over a given distance it travels in the barrel. I would love to have someone prove me wrong though!