It appears that by using the 14.6 inch barrel from the 18 inch barrel you'll be sacrificing approximately 61.862 feet per second (fps). 62 fps may not sound like much, BUT! the 62 fps loss is at the high end of the velocity curve, which means that you will be sacrificing a lot of foot pound energy (fpe); in other words, it takes a heck of a lot more energy to propel a pellet from zero to 604.3 fps (with the 14.6 inch barrel for 11.6 fpe with 14.3 grain pellet) than to propel from zero to 666.2 fps (with the 18 inch barrel for 14 fpe with 14.3 grain pellet). A 2.4 fpe sacrifice is equivalent to 5.14 gr BB fired at 459 fps, which is equivalent to a high power BB pistol CO2, or a low power CO2 BB rifle. To me, that's unacceptable if I will be hunting; especially for survival. All else is aesthetics. I did bar atmospheric conditions, coefficient of friction of pellet to barrel and length of barrel and also very importantly, the compression of the air column in front of the pellet because contrary to popular belief, the compression of an air column of 14.6 inches is NOT the same as 18 inches. The laws of physics are not just really good ideas, and numbers do not lie, but certain manufacturers do lie with numbers.
The quick and dirty equation you can use up to a 24 inch barrel with CO2 as the propellent gas, full sized port, angled and maximum hammer spring countering minimum valve spring is the following:
Ln(2 x Bbl) x (166(+/-12) + 2 x Bbl); Barrel length (Bbl) in inches
For example, a 6" barrel in good tune at optimal temperature of CO2 can have 442 fps +/-30 fps.
Ln(2 x 6) x (166 + 0 + 2 x 6) = 2.48491 x 178 = 442 fps at the mean, median, and mode (in a perfect world).
The +/- has to do with the pellet/barrel coefficient of friction of the static and kinetic frictions, which is usually at a ration of 4:1, more if bevel rounded for skirt to hold for a higher preload to cause a higher initial impulse geometric curve acceleration inside the barrel in the first 3 to 4 inches then the ratio can approach 5:1.
Lastly, the longer the barrel for the same muzzle velocity, the better the accuracy because of the better precision from the lower muzzle pressure impulsion of the pellet as the coefficient of friction from the pellet to barrel goes to zero and said frictional force gets converted to an impulse of increase velocity jump of about 4.5%, which if the crown is not perfectly symmetrical, you will experience it at longer and longer range with what are erroneously called flyers ( a "flyer" is like a miracle where there is no clear-cut explanation).
