I found an interesting post on an Polish forum about the effect of barrel length on pellet velocity. The original is here: Polish Forum
This guy Gruby experimented with barrels of three different lengths on his 16 Joule (12 fpe) Walther LG300 Hunter rifle. The Hunter has an 565 mm (22,2 inch) barrel. With this barrel the rifle shot 239 m/s (784 fps). His Walther LG300 Hunter has a 16 Joule MK2 pressure regulator.
He then changed to a 425mm (16,7″) length barrel of a LG300 7,5 Joule rifle. The rifle then produced 229 m/s (751 fps). He upped the velocity by increasing the hammer spring preload with one turn to 240 m/s (787 fps).
He then installed the LG300 Dominator barrel that is 495mm (19,5″) and the rifle shot 251 m/s (824 fps). He decreased the hammer spring preload with 3/4 turn to get to 240 m/s (787 fps) again.
Then he installed the 565mm Hunter barrel again and got 247 m/s (810 fps). Turned the hammer spring back 1/4 turn and was back at the start with 240 m/s (784fps).
He also shot groups with the different barrels and shows pictures of them. But unfortunately only at 20 meters distance. I could not see any significant effect on accuracy from these different barrels.
This experiment shows that a 140mm shorter barrel (565 mm Hunter to 425 mm LG300) lost 10 m/s (32,8 fps) in velocity at the same setting of the preload on the hammerspring. Going to a 70mm longer barrel (425 mm LG300 to 495 mm Dominator) increased the velocity by 11 m/s at a hammerspring preload that was 1 turn up. Going to an extra 70 mm (495 mm Dominator to 565 mm Hunter) increased the velocity by 7 m/s at the 1/4 turn up hammerspring setting.
For the process of converting a 7,5 Joule LG300 10 meter rifle to a 16 joule (12 fpe) field target rifle this means with that with its standard 425mm barrel it is no problem to get 16 Joule. It just means you have to increase the hammerspring preload by 1 turn clockwise.