I believe I understand what you are asking, and the difference in going from the 24 " chart gun and your 26 " gun is to add 10 f.p.s. vel. for every inch your barrel is longer and subtract 10 f.p.s. for every inch under 24 " . So if your barrel is 26 inches, you would add 20 f.p.s. to the vel.

MET