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Air Rifle Barrel Transverse Vibration Measurement --- On the Cheap!


Steve in NC
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In another thread, experimental physicist (and shooter par-excellence) John C. describes in detail the design, and analysis of data acquired with, an elegant apparatus for optical measurement of springer muzzle angular vibration.  Recommended reading!

https://airgunwarriors.com/community/airgun-talk/air-rifle-barrel-vibration-measurements/#post-49483

Lacking access to anything as sophisticated as John's instrumentation, but still curious about what my springers are doing, I got to wondering what might be possible on a more limited (okay, certified cheapskate) budget.  Here's what I did.

Taking the short-stroke (1cm) velocimeter (VM) used for earlier linear recoil experiments...

image

...I removed its spring, applied a dab of PTFE tape on its magnet to reduce sliding friction (since springers recoil longitudinally as well as transversely --- !!! --- the magnet was going to have to accommodate that with as little resistance as possible), stuck that end of the magnet on the barrel of my 15fpe Webley Tomahawk...

IMG 3327 (2)

...the other end into the coil of the VM...

IMG 3326 (2)

...and fired a shot!

THvib2

Analysis of the results to follow.


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Steve in NC
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Raw AC coupled signal in red, software restored DC in black.  Pellet exit occurs at ~22ms during the first burst of ~1kHz vibration undoubtedly excited by piston bounce.  Y axis units are m/s.

image

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Gratewhitehuntr
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22ms.

Did you wind that coil yourself? The hard way, or drill and laser tach?  We won't think less if it was a shelf part.

 

Next, we'd like to know the travel time of the piston, and actual distance of bounce with various pellets.


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Steve in NC
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Posted by: @gratewhitehuntr

22ms.

Did you wind that coil yourself? The hard way, or drill and laser tach?  We won't think less if it was a shelf part.

 

Next, we'd like to know the travel time of the piston, and actual distance of bounce with various pellets.

Caveman DIY -- just a hand drill adding wire on a plastic bobbin until it looked about right.  Worked out to around 2000t of 40AWG, estimated from coil resistance.

Here's a plot of longitudinal acceleration of the same gun.  Piston "flight time" runs about 10ms duration from 9.5ms to 19.5ms.  Please note that in both transverse and longitudinal plots, the sear breaks and piston acceleration begins at the pretrigger point around 9.5ms, not zero.

image

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Steve in NC
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A c.a. pellet exit time blow-up of the transverse velocity plot shows a maximum velocity peak (at ~20.7ms) of 0.6m/s = 2fps, which if simultaneous with pellet exit, working against an MV of ~925fps, would cause ~ 0.12 degrees of pellet deflection.

Big "if."

image

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Steve in NC
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Transverse displacement -- integrated velocity signal.  Y units are meters.  2.2mm = 0.09"

If we assume the rifle rotated around its center of mass, 0.09" of muzzle-down displacement would equate to about 0.17 degrees of trajectory depression.   Too bad it's in the same direction as the 0.12 degree number for transverse velocity.

Otherwise they'd do a fair job of cancellation -- instead of adding.

image

 


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Bigbore
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Steve,

 What software are you using to collect and then chart the data? Sorry if you mentioned it and I missed it.


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Steve in NC
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Posted by: @bigbore

Steve,

 What software are you using to collect and then chart the data? Sorry if you mentioned it and I missed it.

I found a freeware scope simulator: "Soundcard Scope" that does an acceptable job of collecting VM signals via generic PC audio hardware:  https://www.zeitnitz.eu/scope_en

I did some research into its safety, and the download seems to be clean, but of course I can't guarantee its freedom from malware beyond saying I've been using it without incident.  It's actually pretty flexible.

The scope sim' stores data as spreadsheet compatible .csv files, enabling data massage and plotting.


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Steve in NC
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Major oopsie!

I belatedly realized that, because my Tomahawk was effectively upside down for this test, so are all the data plots!  So wherever I wrote about trajectory depression, please read elevation.

Unless, of course, you habitually hold your springers upside down when you shoot them.

Seriously: Abject apology.


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Steve in NC
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Whenever I read any of John C's or Jim-in-UK posts, I always get a severe case of 2-channel envy.  Today the Amazon delivery guy delivered my cure!   Now I can acquire recoil and pellet exit data simultaneously!  Not a bad upgrade for $20 delivered!

image

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Steve in NC
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PS:  While the UCA-202 is AC coupled, its input timeconstant is 10x better than the other audio adapters I've tried, which makes software DC restoration...

https://airgunwarriors.com/community/airgun-talk/a-scientific-look-into-the-dynamics-of-the-shot-cycle-of-three-spring-piston-airguns/paged/3/#post-48374

...work much better out to 100ms, and so is adequate for accurate springer work.

image

 


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Gratewhitehuntr
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Posted by: @steve-in-nc

Major oopsie!

https://gifimage.net/wp-content/uploads/2017/10/bubbles-trailer-park-boys-gif-10.gif

Gee... yeah... you're right!

Turns out, my brain was only viewing amplitude, auto corrected the orientation, while I was busy completely ignoring the chatter around muzzle movement. IOW, by completely ignoring the entire purpose of you experiment... chart was still fine for viewing the recoil impulse. 🤣 


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Steve in NC
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Posted by: @steve-in-nc

Whenever I read any of John C's or Jim-in-UK posts, I always get a severe case of 2-channel envy.  Today the Amazon delivery guy delivered my cure!   Now I can acquire recoil and pellet exit data simultaneously!  Not a bad upgrade for $20 delivered!

image

First shot acquired by dual channel CODEC -- red trace is Tomahawk (near-muzzle) barrel vib's (negative = upward barrel motion), green is audio of pellet exit picked up by a dynamic ear bud stuffed under the catalog serving as backstop.

image

No more 2-channel envy!


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Steve in NC
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Cometa 400 (14fpe with CPLs).  Upper plot = muzzle transverse velocity in m/s, lower plot = muzzle deflection in meters, pellet exit = vertical line.

image

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Hector J Medina G
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Are we seeing the same thing in BOTH traces?

When you say "vibrations" in the Tomahawk case, is that a Velocity or a Deflection trace?

I assume that the "m's" in the Tomahawk trace are mV, not meters, am I correct?

The whole thing is shaping up....

 

 

HM


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Steve in NC
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Posted by: @hector-j-medina-g

Are we seeing the same thing in BOTH traces?

When you say "vibrations" in the Tomahawk case, is that a Velocity or a Deflection trace?

I assume that the "m's" in the Tomahawk trace are mV, not meters, am I correct?

The whole thing is shaping up....

 

 

HM

Yes and no.  In both Tomahawk and Cometa transverse traces, Y axis scale factors are the same: m/s = meters per second (i.e., velocity), m = meters (i.e., displacement).  Only the Tomahawk longitudinal acceleration plot differs.  There Y = gravities = 9.8m/s^2.

Sorry for any lack of clarity.

For a bit more context, if we assume the Cometa rotates around its CM, I measure approx 0.5m between muzzle and CM, therefore the transverse displacement at pellet exit of ~1.7mm equates to 3.4 mrads = 12MOA of elevation.


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Steve in NC
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PS:  Cometa measured transverse muzzle flip velocity at exit is 0.45m/s, which, if we take MV = 890fps = 271m/s, equates to 0.45/271 = 1.7mrad = 5.7MOA.

Since both muzzle flip and displacement are in the same direction (elevation), they sum to 17.7 MOA of total POI shift from unrestrained recoil.

Applying the same math to the Tomahawk displacement and velocity data, yields (0.17 + 0.12) x 60 = 17MOA = nearly the same result as the Cometa.

Which is as surprising as it is cool!

Note:  Edited for typos.


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Steve in NC
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And now, to be filed under the heading Well, duh: my 16fpe, MV = 955fps = 291m/s, TX200...

Transverse muzzle velocity = 0.15m/s = -0.00052mrad = -1.8MOA,

Transverse displacement = +0.00092m = -0.0018mrad = -6.2MOA

Net POI shift: -1.8 + -6.2 = -8MOA = less than half the magnitude of recoil-dependent trajectory movement seen in either of the two breakbarrels.  

True fact:  I did this test with the TX only after posting the breakbarrel results cited above.  Honest.

image

Edited for typos.


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Hector J Medina G
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Am I to understand that you are adding a velocity and a displacement?

Extremely puzzled about this. I am probably not understanding this correctly. Trying to get what you are doing: it seems you are splitting the TOTAL muzzle movement into a "Displacement" that occurs due to the whole rifle moving around its CoG, and a "Muzzle Flip" that is actually the variation of the muzzle position relative to the rest of the rifle due to the vibrations induced in the barrel by the shot cycle, as an independent phenomenon to the movement of the whole rifle. ¿Correct?

In my simple mind, I do not care about the muzzle "Displacement", as long as it is fairly constant. And I would venture that this is a function of the "hold". A good way to measure this would be to add weights to the guns in different places and see how they react. In the end, whatever you are shooting, even PCP's, need a CONSISTENT hold.

Now, and I emphasize: Again, this is the simple mind of the gunsmith.- If the transverse muzzle's velocity is near zero, then you have a good tune. Otherwise, there are things to change.

But that is just my simple mind. 

On the specific last plot about the TX200: ¿Did you remove the shroud, or are you measuring the displacement and flip at the shroud?

Any expansion of your conclusions would be greatly appreciated.

THANKS!

 

 

 

 

 

 

 

 

HM


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Steve in NC
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Posted by: @hector-j-medina-g

Am I to understand that you are adding a velocity and a displacement?

Extremely puzzled about this. I am probably not understanding this correctly. Trying to get what you are doing: it seems you are splitting the TOTAL muzzle movement into a "Displacement" that occurs due to the whole rifle moving around its CoG, and a "Muzzle Flip" that is actually the variation of the muzzle position relative to the rest of the rifle due to the vibrations induced in the barrel by the shot cycle, as an independent phenomenon to the movement of the whole rifle. ¿Correct?

In my simple mind, I do not care about the muzzle "Displacement", as long as it is fairly constant. And I would venture that this is a function of the "hold". A good way to measure this would be to add weights to the guns in different places and see how they react. In the end, whatever you are shooting, even PCP's, need a CONSISTENT hold.

Now, and I emphasize: Again, this is the simple mind of the gunsmith.- If the transverse muzzle's velocity is near zero, then you have a good tune. Otherwise, there are things to change.

But that is just my simple mind. 

On the specific last plot about the TX200: ¿Did you remove the shroud, or are you measuring the displacement and flip at the shroud?

Any expansion of your conclusions would be greatly appreciated.

THANKS!

HM

I apologize for my lack of clarity Hector.

Actually you're already pretty much correct.  To be a bit more precise, what I'm adding are two separate components of the pellet's vector of departure due to...

1. The transverse component (i.e., angle) of the pellet's velocity in m/s contributed by the muzzle's transverse velocity at the instant of pellet exit, divided by the pellet's longitudinal muzzle velocity, likewise in m/s. 

2. The angle of the bore at the instant of exit due to rotation around the rifle's CM as computed from the transverse displacement of the muzzle in meters, divided by the radius of rotation which I assume to be the distance between muzzle and CM, likewise in meters.

Perhaps a sketch will help -- motion much exaggerated of course.

image

I did not remove the TX shroud, but assumed its motion was an adequately accurate reflection of the barrel's.

As to the importance of either component to accuracy, of course all that matters is, not their magnitude per-se, but rather how they may deviate (e.g, due to inconsistency in hold, etc.) from shot to shot.  If both are perfectly constant, then however large or small they are is irrelevant to accuracy.  However, the larger their average magnitude is, then perhaps the larger the shot to shot deviations that may be expected.


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Hector J Medina G
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Posted by: @steve-in-nc

I apologize for my lack of clarity Hector.

Actually you're already pretty much correct.  To be a bit more precise, what I'm adding are two separate components of the pellet's vector of departure due to...

1. The transverse component (i.e., angle) of the pellet's velocity in m/s contributed by the muzzle's transverse velocity at the instant of pellet exit, divided by the pellet's longitudinal muzzle velocity, likewise in m/s. 

2. The angle of the bore at the instant of exit due to rotation around the rifle's CM as computed from the transverse displacement of the muzzle in meters, divided by the radius of rotation which I assume to be the distance between muzzle and CM, likewise in meters.

Perhaps a sketch will help -- motion much exaggerated of course.

image

I did not remove the TX shroud, but assumed its motion was an adequately accurate reflection of the barrel's.

As to the importance of either component to accuracy, of course all that matters is, not their magnitude per-se, but rather how they may deviate (e.g, due to inconsistency in hold, etc.) from shot to shot.  If both are perfectly constant, then however large or small they are is irrelevant to accuracy.  However, the larger their average magnitude is, then perhaps the larger the shot to shot deviations that may be expected.

Thanks, Steve!

In general I agree that as long as the vibrations/motions are consistent, things should be consistent but, then, the MV also needs to be consistent. And, as we know, it is not.

As a gunsmith, I have seen guns produce single digits ES, and almost zero sSd in 20 shot strings and even 100 shot strings. BUT, next string, something "pops up". EVEN if the shooter him/her self is absolutely consistent on hold, the variance of MV would inject variances in the POD (Point of Departure), as well as the 3D velocity of the pellet.

For a humanly achievable uniform hold then, the most important variation introduced into the pellet's trajectory, and therefore POI given an initial (pre-shot cycle) POA, is the transverse velocity of the muzzle.  In my simple mind, this confirms that the best possible use of the efforts in tuning a gun are in making the barrel vibrate in such a way that the transverse velocity of the muzzle is as close to zero as possible, and that is where the "harmonics tuners" come into place (between quotes because I have been corrected, LOL!). By making the pellet leave the muzzle at the end of most of the excursions through space (in the end, this is also a statistical problem), the transverse velocity is minimized and most of the accuracy / precision falls on the shooter, the hold, and the uniformity of the internal ballistics.

The other way of attacking this problem is to change the pellet exit point in time. Whether with a "Snappy" tune or a "Soft Thud" tune, what we are moving is the Dwell Time of the pellet in the barrel, and therefore the point in time where it leaves the muzzle.

What is fantastic about this whole experiment of yours is that NOW we have an objective means to define what we need to do to achieve a close to optimum tune. AND we can check how good a shot cycle is in objective terms.

So, congrats, well done!

Now, it would be nice to build some sort of "portable" system that can be used in the field. With the shooter shooting his gun, with his pellets, in his position.

😉

As for the TX, I am not so sure I would trust the shroud to follow real closely the barrel or to provide good data. Consider that the barrel itself is about 3" shorter than the shroud, so the real radius is different and it is an important number in the position of the CoG relative to the muzzle, as the hollow shroud cannot be as heavy per unit of length as the barrel/shroud combination. Barrel IS glued to the shroud, so it may not be important.

Thanks, Great work!

 

 

 

 

HM


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Steve in NC
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Thanks, Hector.   I'm happy to have made a contribution to the art that's better than criticism.


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Steve in NC
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It's common knowledge that loose stock screws screw up springer accuracy.  But what objective effect do they have on what the muzzle's doing at the moment of pellet exit?

Here are overlaid plots of transverse muzzle velocity in my long-suffering Cometa 400 guinea pig, with its stock screws deliberately loosened 0.5, 1, and 1.5 turns from snug.

Left hand Y axis units are meters/second.  1m/s = 12.6MOA = 2.6" of vertical spread at 20yds.

They say the exception proves the rule.

image

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Jim in UK
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Excellent result, clearly showing changes in both amplitude and timing. 

I think you have invented a very useful tool for anyone interested in barrel tuning muzzle weights. 


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Steve in NC
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Posted by: @jim-in-uk

Excellent result, clearly showing changes in both amplitude and timing. 

I think you have invented a very useful tool for anyone interested in barrel tuning muzzle weights. 

Thanks, Jim.  I think it's just more testimony to the wonderful versatility of your VM invention!


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Hector J Medina G
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Thanks, Steve!

I agree with Jim, it is an excellent result. BTW, I assume you are loosening the FRONT stock screws, NOT ALL of them.

I did expect the change in amplitude, but the fact that it also changes the timing shows that there MAY be some merit to "pillar bedding" of the actions to the stocks.

For years I have been telling people to "tune" their three screw  (one at the trigger guard and two in the forearm) break barrels by choosing a torque value of the forearm screws, by trial and error, that suits the gun, the shooter and the pellet.

I have not found the same magnitude of a positive effect in four screwed guns.

I firmly believe this is the scientific validation of that experience, at least for the more common break barrels.

Thanks again!

 

 

 

 

HM


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Steve in NC
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Posted by: @hector-j-medina-g

Thanks, Steve!

I agree with Jim, it is an excellent result. BTW, I assume you are loosening the FRONT stock screws, NOT ALL of them.

I did expect the change in amplitude, but the fact that it also changes the timing shows that there MAY be some merit to "pillar bedding" of the actions to the stocks.

For years I have been telling people to "tune" their three screw  (one at the trigger guard and two in the forearm) break barrels by choosing a torque value of the forearm screws, by trial and error, that suits the gun, the shooter and the pellet.

I have not found the same magnitude of a positive effect in four screwed guns.

I firmly believe this is the scientific validation of that experience, at least for the more common break barrels.

Thanks again!

 HM

Correct -- my screw fiddling was limited to the two forestock screws.  Among other considerations, I didn't want to risk a "cocking surprise" when torque from the barrel might disconnect and lift the end of the action completely out of the stock! 

I should have said so from the getgo.

Thanks, Hector, for the kind feedback.

 


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Hector J Medina G
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Credit where credit is due.

Great work!

 

 

 

 

HM


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Steve in NC
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Not all airguns are springers!

What can Jim's invention tell us about PCP Transverse barrel vibrations?

Here's a shot taken with the same Disco I used for a locktime measurement...

https://airgunwarriors.com/community/airgun-talk/so-were-the-cardews-wrong/paged/2/#post-50091

We see a +80mV = 8cm/s upward transverse muzzle velocity at pellet exit, corresponding to only 1.1MOA of trajectory elevation.  Just a simple example of how (even cheap and untuned) PCPs tend to be easier to shoot than springers.

image

 

 


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Steve in NC
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Overlay of Disco muzzle velocity (green - left vertical axis units = decimeters/s), displacement (black - right vertical axis units = meters), pellet exit sensor (red).

Interesting how displacement traverses zero ca. pellet exit.

image

 


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nervoustrigger
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Good stuff!

My inference from the graphs is that the Disco, with this particular pellet and state of tune, will exhibit a relatively high degree of lateral dispersion on target (horizontal stringing).  The pellet is arriving at the muzzle when the muzzle’s displacement is roughly zero...its natural at-rest position.  That sounds good at first blush but it means the pellet is exiting while the muzzle’s velocity is highest, meaning the muzzle’s position is changing rapidly.  Thus the next pellet which inevitably arrives at the muzzle at a slightly different time, will exit with the muzzle in a different position.


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Steve in NC
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Posted by: @nervoustrigger

Good stuff!

My inference from the graphs is that the Disco, with this particular pellet and state of tune, will exhibit a relatively high degree of lateral dispersion on target (horizontal stringing).  The pellet is arriving at the muzzle when the muzzle’s displacement is roughly zero...its natural at-rest position.  That sounds good at first blush but it means the pellet is exiting while the muzzle’s velocity is highest, meaning the muzzle’s position is changing rapidly.  Thus the next pellet which inevitably arrives at the muzzle at a slightly different time, will exit with the muzzle in a different position.

Agreed.  But on the other hand, because transverse velocity is at a peak, it's relatively constant around the nominal moment of pellet exit.  Therefore pellets that arrive slightly earlier or later (e.g., because of different starting pressures) won't receive a different "kick" from the sidewise movement of the muzzle onto different vectors of departure.

This is easier to see if we zoom in on PET (Pellet Exit Time?).  Like so.

image

PS:  The sharp eyed (i.e., picky people) may have noticed that this plot puts pellet exit about a millisecond earlier than the one in the other thread.  This is due to the scope trigger being taken from the VM signal here at the muzzle, but there at the breech.  This "missing millisecond" represents the delay taken by the mechanical commotion that starts at the breech to rattle down the barrel and (eventually) arrive at the muzzle.  The only way to avoid this kind of effect would be to have a separate independent trigger channel.

Which, sadly, I ain't.


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Steve in NC
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Candid photo of (cheap) barrel vibration rig.   Winding VM coils on 1/4" ID polycarbonate tubing turns out to be a much more flexible medium than sewing machine bobbins stolen when the wife isn't looking!

image

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