SpudFiles

Hopefully you can get GGDT working, it'll answer all your questions.

Here is what I get. Had to make a few guesses about the ammo mass (used 100g which is probably too heavy), valve parameters etc. The horizontal axis is the barrel length, the vertical the velocity at that barrel length.

As you can see, the optimal barrel length is about 70". But, chopping the barrel length in half to 35" has very little affect on the muzzle velocity.

The inner diameter of 1.5" pipe is 1.61", not 1.5, and he said the bore would be 0.75", not 1.5".

By these specs, he could use a barrel more than 12' long and still be gaining performance.

As for practicality, use as long a barrel as is practical.