Depending on your valve, chamber and projectile factors the ideal barrel length can be found. Do not be fooled in GGDT looking at the projectile still accelerating at the muzzle. I fell for this too as it implies longer will permit a longer acceleration with higher muzzle velocity. The reality of a longer barrel resulted in LOWER acceleration with it still accelerating at the muzzle for a net LOWER muzzle velocity.
To utilize a long barrel, a large diameter is required along with a large chamber and high pressure. A small diameter has too much flow restrictions and a long barrel hinders performance.
In the forum, I have covered this topic before with in barrel acceleration measurements to support the theory. A 10 foot barrel for a 1.5 inch combustion spudgun is too long. A 5 foot barrel will outperform it. A 96 foot long barrel regardless of chamber size on an advanced combustion will underperform. The flow restriction and projectile friction will severely cut performance. The projectile will cease accelerating long before it reaches the muzzle.
Here is a graph with a scope picking up a magnet in a projectile as it traveled a 10 foot barrel. By measuring the diminishing time between pulses you can see the delta velocity or acceleration. Note the last 4 feet of this shot shows little to no acceleration. After trimming to about 7 feet the acceleration was much greater as well as a higher muzzle velocity.
This graph does not show the entire 10 feet. It shows the last few feet. the muzzle coil pick up has reversed polarity to show it is the last pick up.
