Should you index your barrel?

Forums General Discussion Should you index your barrel?

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    Michael
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    Find out!

     

     

    • This topic was modified 4 days ago by Michael.
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    Oregun
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    Wow…  what a difference!  Very well explained.  I wish I could do that with all my rifles.  Thanks for sharing, Michael!

    –Carl

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    biohazardman
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    I have my barrel indexed but did not think about the barrel twisting when tightened down so I will look into that as my gun shoots a bit farther to the right than I would expect at longer ranges.  Thanks for the vid and the other things you do for our hobby/sport/diversion/sickness/addiction. ;^)

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    Michael
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    biohazardman

    I have my barrel indexed but did not think about the barrel twisting when tightened down so I will look into that.  Thanks for the vid and the other things you do for our sport/sickness/addiction. ;^)

    🤘🙏

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    Therealld
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    I began indexing airgun barrels decades ago, when it was generally much more difficult to accomplish. But the REASON for indexing back then was to change the harmonics of the barrel improve accuracy, and not so much to change relative point of impact.  Now the harmonics issue is a ticklish one, since many barrels don't respond with improved accuracy … but some DO!   Even when accuracy does improve, its generally not a huge improvement that seems well worth it to most regular airgun owners.

    I have indexed literally hundreds of barrels, and see some that improve shot precision as much as 1/4", or on rare occasion, maybe a bit more, but more often, there is not a significant difference (significant difference in scientific parlance means not scientifically repeatable, or in other words not out of the normal distribution).

    But I DO continue to index most new barrels I install, and because even relatively tiny improvements in accuracy are important for me in the match guns I play with.  A gun that shoots consistent 50 yard groups of around 1/2" to 9/16" that improves 1/16" or better is good for me, and one that shoots 1/8" better is a real Gem that might enable me to shoot better scores.

    I know for a fact that some barrels are not bored perfectly concentric the the outside diameter, and therefore have some areas with thicker walls on one side than the other.  Its for this reason that harmonics vary as the barrel is rotated, since the effect of gravity, and that of recoil impulse of firing can, and does change depending on barrel orientation IF the barrel is not precisely symmetrical inside and out.  Even a barrel that has a bend in it, or one that DID have a bend that was straightened will be a bit stiffer in one axis than another … and will likely vibrate with a different frequency and amplitude in axis.

    I have found a barrel that is perfectly straight ( I prefer to straighten new ones so the exterior is straight, though I realize most barrel makers try to straighten by looking up the bore) that shoots to differing poi as its indexed usually shoots best (tight groups) with the lowest poi position, though sometimes its the highest position, or a push.  Since I have really tested so many barrels, I now generally just index to find the lowest point and thats that .. but its not about centering the impact to the scope crosshair, but but for tighter grouping.

     

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    Michael
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    Therealld

    I began indexing airgun barrels decades ago, when it was generally much more difficult to accomplish. But the REASON for indexing back then was to change the harmonics of the barrel improve accuracy, and not so much to change relative point of impact.  Now the harmonics issue is a ticklish one, since many barrels don't respond with improved accuracy … but some DO!   Even when accuracy does improve, its generally not a huge improvement that seems well worth it to most regular airgun owners.

    I have indexed literally hundreds of barrels, and see some that improve shot precision as much as 1/4", or on rare occasion, maybe a bit more, but more often, there is not a significant difference (significant difference in scientific parlance means not scientifically repeatable, or in other words not out of the normal distribution).

    But I DO continue to index most new barrels I install, and because even relatively tiny improvements in accuracy are important for me in the match guns I play with.  A gun that shoots consistent 50 yard groups of around 1/2" to 9/16" that improves 1/16" or better is good for me, and one that shoots 1/8" better is a real Gem that might enable me to shoot better scores.

    I know for a fact that some barrels are not bored perfectly concentric the the outside diameter, and therefore have some areas with thicker walls on one side than the other.  Its for this reason that harmonics vary as the barrel is rotated, since the effect of gravity, and that of recoil impulse of firing can, and does change depending on barrel orientation IF the barrel is not precisely symmetrical inside and out.  Even a barrel that has a bend in it, or one that DID have a bend that was straightened will be a bit stiffer in one axis than another … and will likely vibrate with a different frequency and amplitude in axis.

    I have found a barrel that is perfectly straight ( I prefer to straighten new ones so the exterior is straight, though I realize most barrel makers try to straighten by looking up the bore) that shoots to differing poi as its indexed usually shoots best (tight groups) with the lowest poi position, though sometimes its the highest position, or a push.  Since I have really tested so many barrels, I now generally just index to find the lowest point and thats that .. but its not about centering the impact to the scope crosshair, but but for tighter grouping.

     

    If i were to choose my indexing position based on group size, it would probably be my 3 o'clock position (which is the upper right group in the picture). 

    The STX barrels only allows indexing to a certain point.  There's no way to index it exactly (yet).  Plus, unless you use a torque wrench, the pressure/harmonics you apply to the liner could be different. 

    I'd like to see FX add a few simple things to make indexing easier.

    1. Add a tiny grub screw somewhere on the sleeve (or barrel, whatever they call it)
    2. Add a few (maybe 8) tiny indents on the liner for the grub screw to hold it in place.  They could even be numbered so that you could look through the grub screw hole to view the #.
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    2fast2furious
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    Wouldn't it be liner index? Cause fx called them liners, not barrels. You actually turn the liners around, not the actual barrel selves. 

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    dwilson050
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    Well done Michael! Thanks for the video.  -Dave

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    Michael
    Keymaster
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    2fast2furious

    Wouldn't it be liner index? Cause fx called them liners, not barrels. You actually turn the liners around, not the actual barrel selves. 

    For the STX systems you could refer to it as "liner" indexing.  I also showed & described how easy it is to do with the EDgun Leshiy (traditional type of barrel without transfer port).

     

    • This reply was modified 4 days ago by Michael.
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    Centercut
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    Thanks Michael, good video. One thing to note, not all FX STX liners rotate that easily in the barrel. I did this indexing on my .22 Crown about a year or so ago, and the liner is so tight that it doesn't turn when I install the retainer nut. I'd assume that some are tighter than others, so not all will rotate with the retainer when installing.  Owners need to check theirs and if they rotate easily follow your advice, but if they are in there firm just install without indexing the retainer nut. 

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    Franklink
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    I have two guns that allow for indexing of barrels. One is actually an LD creation (he commented above), a USFT. The other is my Raptor .20. I, too, have found that groups can be improved or worsened as the barrel is rotated. Many designs do not allow for it, but accuracy can be improved enough to warrant testing on a gun where it is possible. 

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    Longbrownbox
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    You may not be an airgun builder and you may not be a tuner but you do have a wealth of knowledge and I pay attention. Great info. Indexed my barrel last Saturday. I had similar results. Thx Chuck M

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    Motorhead
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    When I re-barrel a gun, before transfer port is drilled or flats milled  ( Dislike thimbles ) the barrel is placed within receiver and rotated while held in a machine rest / vise.

    When the bore clocks at 12:00 it is marked as such. Removed with transfer port being drilled at 6:00, anchor flats etc at 12:00

     

    Thimbles while sounding like a great idea are troublesome in many ways and simply avoid them when possible.

     

    Too each there own … ultimate accuracy wins the day …. choose wisely.

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    Bigragu
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    Michael

    Therealld

    I began indexing airgun barrels decades ago, when it was generally much more difficult to accomplish. But the REASON for indexing back then was to change the harmonics of the barrel improve accuracy, and not so much to change relative point of impact.  Now the harmonics issue is a ticklish one, since many barrels don't respond with improved accuracy … but some DO!   Even when accuracy does improve, its generally not a huge improvement that seems well worth it to most regular airgun owners.

    I have indexed literally hundreds of barrels, and see some that improve shot precision as much as 1/4", or on rare occasion, maybe a bit more, but more often, there is not a significant difference (significant difference in scientific parlance means not scientifically repeatable, or in other words not out of the normal distribution).

    But I DO continue to index most new barrels I install, and because even relatively tiny improvements in accuracy are important for me in the match guns I play with.  A gun that shoots consistent 50 yard groups of around 1/2" to 9/16" that improves 1/16" or better is good for me, and one that shoots 1/8" better is a real Gem that might enable me to shoot better scores.

    I know for a fact that some barrels are not bored perfectly concentric the the outside diameter, and therefore have some areas with thicker walls on one side than the other.  Its for this reason that harmonics vary as the barrel is rotated, since the effect of gravity, and that of recoil impulse of firing can, and does change depending on barrel orientation IF the barrel is not precisely symmetrical inside and out.  Even a barrel that has a bend in it, or one that DID have a bend that was straightened will be a bit stiffer in one axis than another … and will likely vibrate with a different frequency and amplitude in axis.

    I have found a barrel that is perfectly straight ( I prefer to straighten new ones so the exterior is straight, though I realize most barrel makers try to straighten by looking up the bore) that shoots to differing poi as its indexed usually shoots best (tight groups) with the lowest poi position, though sometimes its the highest position, or a push.  Since I have really tested so many barrels, I now generally just index to find the lowest point and thats that .. but its not about centering the impact to the scope crosshair, but but for tighter grouping.

     

    If i were to choose my indexing position based on group size, it would probably be my 3 o'clock position (which is the upper right group in the picture). 

    The STX barrels only allows indexing to a certain point.  There's no way to index it exactly (yet).  Plus, unless you use a torque wrench, the pressure/harmonics you apply to the liner could be different. 

    I'd like to see FX add a few simple things to make indexing easier.

    1. Add a tiny grub screw somewhere on the sleeve (or barrel, whatever they call it)
    2. Add a few (maybe 8) tiny indents on the liner for the grub screw to hold it in place.  They could even be numbered so that you could look through the grub screw hole to view the #.

    Grub screw idea is good, or even a lightly springed ball detent like what’s on a ratchet on the outer sleeve to catch those indents on the STX liner. Just something to hold it in place as you tighten up the lock nut.

     

    I predict with as popular indexing is becoming, a knurled end cap that allows for click turning of the inner STX liner will be seen in the future, sort of how Browning came out with their BOSS system back in the 90’s,  of dialing up their rifles at the muzzle for harmonics and taming down the rapport.

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    8thman
    Participant
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    Thanks for the video MICHAEL, Did you try on different power levels and have the same result but with slower velocity? For example at cooler temperatures my impact slows 10fps or more for every 10 degrees drop in temperatures. Just wondering if there would be a difference in indexing based on speed.     Thanks again MICHAEL 

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    Therealld
    Participant
    Member

    Mike,

    Be careful with group size when the sample is very small, such as one five shot group.  When I used the term Normal Distribution, it means even the widest shots would likely fall within that deviation if a large number of shots were fired.    Typically, the SMALLEST sample size that is usually considered a "large number" is "over thirty".   I have tested many barrels by firing thirty one ( 31) shot groups, and must say they typically grow much larger than most five shot groups from the same conditions.  I did literally weeks of testing in my new 51yard tunnel with airguns, and learned the following:

    a. 25 shots is just about as good as 31 shots from a decent gun and known decent pellets to decide the "Normal Group Distribution" of that rig under ideal conditions.  And 5 five shot groups can do it too, IF you fire at identical bulls using the center aimpoint, and measuring wide shots from the bull center.  I could do this since I had no wind, and it allowed some time between groups and did not obliterate the aimpoint so easily.  Even ten three shot groups worked well, so long as conditions were as equal as possible, and wide shots were measured from the center of the bull.

    b. Each new condition needs to be considered as a new process, esp different pellet types, significant velocity adjustments, and even … including different barrel,  breech or Crown mods, indexing, muzzle devices such as weights or silencers etc.

    Yes, I mean if you want to use the smallest group of  your test to lock in on index position, you should verify it really IS the tightest by firing more shots from each position.  I admit I don't do that anymore because I tested for this so much with many many barrels that I was convinced that the six o:clock position was good.

    BTW, here is my most logical reason besides test results for selecting the six o:clock:   It is well-known that lighter and heavier ammo tends to throw shots higher or lower, and also faster and slower ammo does the same (light/fast ammo since it usually throws shots lower at close range and higher at longer).  So it is understandable and expected that 50 yard groups would tend to have more variation in the vertical dispersion that horizontal, due to normal pellet and velocity variations … but horizontal shifts are likely caused by the barrel not wanting to shift vertically as easily, most likely due to variation in stiffness in the lateral mode. 

    I think a barrel that has wall thickness or other stiffness issue, such as stress due to straightening will typically favor amplitude changes in a particular direction.  Imagine a barrel that has been bent, then straightened up and down a couple times …. this produces stress that tends to make the barrel stiffer in that direction, and more likely to move side to side … but if it is then turned 90 degrees, it will resist horizontal movement, which is (to me) better.

    • This reply was modified 2 days ago by Therealld.
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    Therealld
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    Thimbles have many issues, and they cropped up because of the REAL culprit …. the transfer port entering the barrel itself!  I have seen the transfer port to be the root cause of most gun accuracy problems, second only to poorly set up silencers (a shroud is usually intend to be a silencer as well).

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