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Thread: Barrel length question

  1. #1
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    Barrel length question

    Most air rifle BR guns have barrels in the order of 24 inches or so. I'm thinking that shortening a barrel down to around 19 inches will reduce the shot time (time the pellet spends in the barrel) and improve accuracy since there's less time for the gun to jump around during the shot cycle. I'm aware that I will lose some velocity by doing this, but I think I can deal with it. Has anyone experimented with this?

  2. #2
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    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
    Last edited by AZUARO; 11-18-2017 at 08:34 PM.

  3. #3
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    Paul:

    I forgot to mention that if you decide to cut your barrel, do it from the breech side (loading or chamber side)...Many barrels have a choke and tons of them have a very gradual choke that you can not tell or feel unless you air gauge it...

    Forget about pushing a pellet through the chamber forward and "feeling the push and tightness" for determining if there is a constriction (choke) or not, this works OK in some barrels but many times it is not felt and If you cut the choke section you basically ruined that barrel's precision...The now short barrel may shoot OK, but it will shoot much better if the choke is left undisturbed.

    Hammer forged barrels ALL have either one: A tight choke or a very gradual one just enough for the manufacturer to be able to pull out the mandrel...

    Regards,

    AZ
    Last edited by AZUARO; 11-18-2017 at 08:39 PM.

  4. #4
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    AZ,
    Thank you for your technical explanation of the various factors that affect the operation of airguns. I never considered most of these in my quest for better accuracy, and you are most correct in that a change in one parameter for the “better” can make something else worse.
    My thoughts about barrel length was merely an attempt to control or reduce the vibration of the barrel during the shot. Most, if not all, airguns use a fairly small diameter barrel cantilevered from an aluminum receiver. A free floating barrel is desirable for accuracy especially for powderburners, but those have steel receivers and heavy barrels. While I have not seen any high speed photography of a PCP airgun barrel during the shot cycle, there must be some vibration or coreolis effect as well as recoil as minute as it is. Therefore, I felt that shortening the barrel would minimize this and have less effect on pellet flight by changing the vibratory frequency. I may be full of you know what, since I have no technical data to back this up. Ideally, wouldn’t it be better, at least for benchrest rifles, to use a heavier barrel and a more robust receiver attachment method? Inserting a thin 24 inch barrel into an aluminum receiver about 1 ½ inch seems too delicate to me. As you said; MASS, MASS.
    Another thing that I feel could be improved is airflow efficiency within the gun. The usual valve, regulator, transfer port system should be studied. Pellet quality presently, is a concern, and I spend lots of time sorting these, so-called “select” pellets. Has pellet design reached its zenith? All these things cost a lot of money to research and, unfortunately, can only be done by the tinkerers like us. Most airgun builders are small and don’t have the kind of resources to fully research these things.
    Again, thanks for taking the time to present such a detailed explanation of airgun operation. You don’t usually find this on the forums.

  5. #5
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    Paul:

    "wouldn’t it be better, at least for benchrest rifles, to use a heavier barrel and a more robust receiver attachment method? Inserting a thin 24 inch barrel into an aluminum receiver about 1 ½ inch seems too delicate to me. As you said; MASS, MASSMASS"...

    Yes Paul, most guns are designed with aluminum frames, and the way barrels are attached is very convenient but as you say delicate...Nevertheless, you find more robust actions like RAW and others that have heavier duty frames. If I were a manufacturer I would try to make this point(Barrel/action attachment) much more robust, but in reality for Benchrest and Field Target shooting at 12 ft./lb. and even 20 ft./lb., these actions and current barrels with profiles in the neighborhood of 16 mm (.630") do the job very well...Heavier calibers and energy levels are another matter but we are just seeing these new calibers and products emerging lately and manufacturers are constantly innovating because there are many players competing for a sale.

    Another alternative to MASS is to STRENGTHEN your barrel ...You can start with a smaller contour lighter barrel and PRE-TENSION it, If you do it right (not with your Lowe's or Home Depo's components) you can achieve very good results...You can also also use a smaller contour barrel and sleeve it and then fill the gap with dampening materials, a strong epoxy like Acraglas or a similar hard and strong filler for added strength, etc ...We have done some of that successfully for aerospace components and there is no reason why it wouldn't work the same in barrels...So again, there is a lot out there to play with and all it takes is people with a little understanding on how things work supported with some ingenuity and creativity...

    About having reached the "Zenith" with our current pellets....A BIG NO, absolutely not...Pellets are actually the weak link in the chain and there is a lot to be done in this field..We need more aerodynamic well designed forgiving pellets, what you see in the accuracy circuits (BR and FT) is the best of what is being produced, but it is a fact that pellet manufacturers are not pushing the envelope and investing in R&D for producing better pellets, maybe because they sell anything and everything they produce...

    In the USA and countries where LEAD in being pushed out because of its toxicity, there are no incentives for new players to produce pellets...Tungsten and other materials considered environmentally friendly are too expensive for this use.
    I would like to see some other alternatives with better technologies...If you look at what top shooters are doing with their pellets to make them shot, it is a very time consuming pretty boring process: Cleaning/washing, measuring heads, weighing, checking skirts and pellets for defects, etc. takes just too much time, is monotonous and boring, and I believe that we shooters are doing the manufacturer's job..It shouldn't have to be this way with today's technologies.

    Redesigning air regulators-ports, etc. is in constant development...I actually believe that Regulator/port/timing are excellent technologies for precision...Much better than for example working in making a precise.22 Rimfire cartdridge-gun where you have more variables out of your control...You or I can achieve lower velocity spreads with current PCP technologies much better than trying to do it with a rimfire rile where you depend on a primer substance, firing pin indentation, case (brass) thickness and hardness, etc..Look at it this way: Regulators, ports and air valves plus a good timing are the equivalent of what is behind the lead bullet of a .22 RF cartridge...Air guns with regulators have a sweet spot which is the ideal setting where the gun shoots the best, this perfect or almost perfect "Timing Point" can not be reached in a rimfire simply because as said, you depend on the inconsistencies of the components the manufacturer used for making the ammo...I rather work on the consistency (pellet velocity) of a PCP air gun that doing the same for a a black powder, rimfire or centerfire gun.

    Regards,

    AZ
    Last edited by AZUARO; 11-17-2017 at 01:49 AM.

  6. #6
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    Paul:

    A friend for whom I did some work on his gun emailed me and brought up a question that I thought would be interesting as complement to the thread…It is again long but we may have audience that needs detailed explanation.

    The Question:
    “AZ, I read your reply to Paul at the Benchrest site and found it illustrating, as you know, I am the kind who doesn’t understand technical matters but enjoy pulling the trigger..If I don’t remember wrong, the barrel on my Steyr was 22 inches long (55 cm. factory barrel) and did not shoot anything better than average 1/4 and 1/2 inch groups at 25 and 50 yards.. then you cut it to 18 inches and the groups were cut in half…Will you please explain the improvement? “

    The answer is simple…Your barrel was not straight and not concentric and any change towards alleviating one or both of these imperfections would improve precision dramatically…The worse a barrel is the more notorious are the improvements.

    When a barrel is not straight but is concentric, indexing that barrel to either 12 or 6 o’clock positions usually provides potential for improving precision…In reality 12 & 6 are both starting positions, you will then have to fine tune them…

    When a barrel is not concentric, indexing may help up to a certain point but that barrel is condemned to be a BIG LOOSER…Dealing with non-concentric axial harmonics and pressures are a nightmare almost impossible to control and not worth the effort, in these cases GET A NEW BARREL looking for maximum concentricity and straightness.

    Now, when a barrel is not non-concentric and is not straight, that barrel is not only a looser but is only good as a paper weight…

    Your barrel was of the last kind: Non-Straight and Non-Concentric…

    We discussed that and I proceeded to do several minor surgeries: Straightened the barrel to the point where breech and crown ends were as close as possible…We were at my home’s shop and I didn’t have any other tools but range rods for measuring both ends and a rubber mallet…I used “V” wooden blocks and crudely did my best…

    Straightening so breech and crown ends are close DOES NOT mean that the barrel is straight by any means but it gets you closer…

    Then I turned the barrel in between centers, forgot to use crown savers and ruined the crown, but re-crowned and re-cut the loading port nicely LOL…I basically “cleaned” and not really turned much of the outer surface to make it concentric to the bore… I didn’t want to fall into having to machine a sleeve and then finding a way to attach it to the breach end in order to fit the Stey’s 16 mm action...Fortunately it turned out satisfactorily concentric…

    I then re-measured and cut a non-concentric stretch and left the barrel at 18” and tested it…Yes, your barrel improved 100% or better, but even after surgery that barrel did not get it into the precision standards that a good straight concentric barrel has to start with…A straight concentric barrel does not respond to indexing because there is nothing to improve, it shoots outstandingly well in any position.

    Bottom line is that yes, shortening your barrel improved it much, but this was not because of making it short but because of alleviating some of the issues that were causing big problems affecting accuracy…

    I have stated before and I still believe that indexing and other remedies as I used for your gun can improve BAD barrels or maybe fine tune some other good barrels, it is OK to do it if you want to do it; but in a crooked barrel, neither indexing nor shortening nor turning it concentric is going to make that barrel a killer. Spend your time and resources finding that good straight concentric barrel that is out there somewhere, a straight barrel does just that: IT SHOOTS STRAIGHT and hits where you aim...Or pay the premium that gun manufacturers charge for their premium guns that basically need very little tweaking…These manufacturers have done the job of finding a good competitive barrel for you and what they charge for their services will make you happy and save you many migraines.

    Best regards,

    AZ
    Last edited by AZUARO; 11-20-2017 at 11:33 PM.

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