https://www.pyramydair.com/blog/2017/12/crosman-2400kt-co2-air-rifle-part-11/
Parts 10 & 11 deal with barrel length.
What he calls the "power valve" is a base 2240 valve. The other is a 2300 target pistol valve which is a lower power/higher shot valve.
Re 13xx the numbers will vary but the principle is the same. Longer barrel=more power/FPS.
Accuracy is determined by the quality of the barrel, not the length.
might finally put the old 18" is the optimum barrel length to bed quote you see so often