Paul:
I am not very good at explaining concepts but let me give it a try...Long but stay with me because I will only touch a few implications of your question and maybe some newbies will learn something productive.
Barrels in rifles used to be "Standard" at 16 inches and even shorter when gun manufacturers were not engaged in the velocity race, then 19 inches became pretty much the standard and for the last decade or so it has been anywhere in between 16 and 20+ inches depending on the manufacturer and intended use of the gun ...
I don't believe that a Shorter Barrel" necessarily implies better accuracy as you state and let me explain why: That concept which is in reality a myth has its origin in guns (powder and air) used in shooting disciplines where the shooter actually wanted the projectile to leave the barrel as soon as possible after he pressed the trigger....Yes, for this effect a shorter barrel and a faster lock time were and are highly desirable...but the reason behind this is that the shooter is shooting in a standing position or in a position that has human movements (kneeling, seating, 3 position, etc.) after pressing the trigger. The ideal scenario is the bullet exiting the barrel exactly at the time when the shooter presses the trigger, but this doesn't happen unless you are using a laser beam and not a bullet...
In these cases where human movements are involved, the shorter barrel adverse effects are less that than the inaccuracy caused by human movements...This is the one and only reason for this myth....
In Benchrest shooting, neither a fast lock time nor a short barrel have a substantial impact on accuracy, a 24 or 26 inch barrel that is straight with good bore dimensions and is concentric, is going to be as accurate as an 18" or 16" barrel with the same characteristics assuming that you tune your gun accordingly...
Proof of this said is the fact that ISSF guns at top levels all use the 16 " standard barrels with very fast lock times ...Shooting a standing position at 10 Meters, your brain and finger shot the "X" ring, but your movements afterwards moved the point of impact to a 9 or worse depending on how much you moved after pressing the trigger and depending on how long the bullet stayed inside the barrel....
By comparison, some Benchrest shooters at top level use barrels from 18-19 to 24 + or - inches, and I have even seen longer 26+ inch barrels shooting outstandingly well...A very long barrel has diminishing returns but this is not part of the topic here...Some shooters use heavier hammers for BR guns which by nature means slower lock times with low frequency low amplitude vibrations vs. the opposite for lighter hammers, but these shooters have usually been playing with the "Timing" of the gun which involves how long the valve stays open & the velocity at which the air is flowing through he ports depending on their size...Some shooters do this knowing exactly how to do it and some others end up with good results doing it intuitively by trial and error.
I have seen some gun manufacturers testing 16" and even shorter barrels shooting BR and FT....First and foremost they have resolved the shot count number/shorter barrel issues by supplying a high capacity-high pressure air cylinder at close to or 4500 PSI , they also are using pressure restriction devices just before the regulators so the regulators or valves don't get stuck or fail as a consequence of the very high 300 BAR pressures...They have also adjusted BAR pressure & hammer strikes and timed their guns accordingly...
To my perspective these efforts are focused towards increasing rigidity, reducing vibrations and bullet time inside the barrel at the expense of higher frequency vibrations natural in a shorter barrel that are augmented when using a higher BAR regulator and/or higher air velocity though the ports as well as when a harder hammer strike is used...We have done much research on vibrations and vibrations control at the lab and this has been our experience, but other people may disagree...
Nevertheless, it is a fact that a shorter barrel would only tend to be more accurate than a longer barrel if the MASS is the same or higher....Put a 24" barrel that weighs 3 pounds vs. a 18 inch barrel that weighs 8 pounds maintaining all other variables equal and most likely the shorter barrel will be more accurate...Again, MASS, MASS, MASS is a best friend to accuracy, but in your question I am assuming that the barrel contour will remain the same for both barrel lengths
This last topic also implies that a barrel vibrates the most (more abruptly) at the moment when the projectile starts moving forward of the chamber, then these vibrations (harmonics) tend to stabilize and "calm down" as the pressure inside the barrel diminishes and as time passes,...The more MASS, natural in a longer barrel vs. a shorter one of the same contour produces less damaging vibrations...Again, MASS for less vibrations and longer times before the bullet exits the barrel for allowing the stabilization of vibrations, both mean better accuracy....If any vibrations are present, you want them with low amplitude and low frequency.
Another important issue...A shorter barrel will induce more "Push" to the projectile as it leaves the barrel...The expanding gas will have much more velocity after leaving the barrel because the pressure has to be higher inside a shorter vs. a longer one for maintaining the same projectile velocity...The higher that "Push" to the projectile the more the inaccuracy potential and this effect is an exponential factor directly proportional to the quality and defects in your bullets/pellets...
So then a bad batch of pellets are MUCH MORE inaccurate in shorter barrels than they are in longer barrels shooting at the same velocity just because of that final push while exiting the gun, this is a fact and not a statement and in this particular case MASS does not help....Our 8 pound shorter barrel mentioned above WILL NOT shoot any better than our 3 pound 24" barrel...So we can conclude that it is a disadvantage to use a shorter barrel and a bad batch of pellets vs. using the same bad pellets in a longer barrel....Given the quality of our pellets nowadays, a smart benchrest shooter would not use a short barrel unless he can be sure all of his pellets are excellent.
All of these issues presented and many more omitted but related to your question are like everything else, there are pros and cons for anything and for everything...Make your decision based on the intended use of your gun...If you require more shots per fill given a same air cylinder capacity and pressure, then get a longer barrel and set your internals to a lower BAR setting for more efficiency on your regulator and adjust hammer weight and strike...If you need a survival short gun use a shorter barrel, but be aware that for maintaining the same velocity you will need a heavier hammer/stronger spring and maybe more BAR pressure and definitely more volume or air at more velocity per shot, which means more VIBRATIONS which are one of the most destructive forces in accuracy.
Summarizing your question & my answer: Think of it this way: If a shorter barrel meant more accuracy, hunters, target shooters and all of us would all be shooting stocked pistols with scopes...
Regards,
AZ
