TargetTalk

Eric,

I'm a physicist, and I'm trying to understand what the oscillations are that you are damping out with a tuner. I can easily see breathing mode (radial) oscillations starting, but I'm darned if I see how to excite something perpendicular to the barrel axis. I guess it must happen because no barrel is perfectly uniform along the length or around the circumference.

And I'm so ignorant of small bore that I haven't a clue how big the vibrations are. Are you talking of something on the order of 0.1 ring or 1.0? I guess the forces involved in air rifle are far too small to excite a steel barrel on an AR. If you can point me towards something to read, that would be great.

This is not really directed just to Eric; if anybody else can help me, I would be grateful.

Thanks!

pete z

It's all to do with the axial force of recoil not being reacted in line with the bore axis. Also asymmetry of the receiver contributes, because asymmetry causes it to flex as it stretches. These, and other, minor effects induce multiple modes of vibration that propagate along the barrel as transverse displacements. They are of small amplitude in smallbore rifles, but are rather bigger in naval guns(!), in which the subject has been extensively researched.

Whether 'tuners' damp the induced vibrations or just lower the natural frequency of the system is a moot point.

Over to you Eric...

On another site there is much discussion regarding bore curvature and how the orientation of the barrel effects the grouping ability of the rifle. The suggestion is that a slight downward curve to the bore produces the optimum results with minimum horizontal dispersion to the groups and vertical stringing due to varying velocity of the round being the dominant source of error.
I'm hoping to see some results from Geoff Kolbe's rig sometime this year. http://www.border-barrels.com/articles/ ... _tests.htm

I'm sure that altering the natural frequency of the barrel is part of the reason for the tuner - I'm a musician and the way you tune a Fender Rhodes electric piano is by moving a weight along the "tine bars" to change their natural frequency. Also by adjusting torque on the "tone bars" (sort of like the bedding bolts) you can alter the balance of harmonics (the tone of the note).

There is an interesting analysis of all of this on Varmint Al's site where he does a Finite Element Analysis of the motion of various barrels and the compensating effect of slightly changing the point at which the bullet exits the muzzle. http://varmintal.com/a22lr.htm

I've also seen is suggested that the ideal situation is where the downward curve of the bore corresponds to the trajectory that the bullet would follow if was in free air - and that adding weight to the muzzle end produces that profile... Hmmmmm, could a small weight cause any significant deviation I wonder?

And then there is the Jeffrey Madison stock which tunes the rifle by damping the receiver while clamping the barrel and supposedly produces an identical effect to adding weight to the muzzle. Sort of like the Fender Rhodes again but altering the harmonic balance rather than the fundamental frequency.

I wish someone had the time and money to do a definitive smallbore study - currently there seems to be a lot of supposition but little hard evidence.

Theories abound about how tuning really works, but nobody has proven a thing. Maybe never will. Just go over to some of the rimfire benchrest forums and read the arguments either way to your hearts content.

All I know is it works and the why doesn't really concern me too much. I'm way more of a practical engineer than a theorist. Results on target matter to me much more than the number of dead snakes (integrals) written in a notebook.

I also don't think you are "damping" vibrations as much as adjusting them to get improved group results.

Eric U

I was waiting for Kiwi to chime in! We had some great chats on this subject a couple of years ago. Time flies.

One of the related issues is something called "shot ping" where certain smalbore rifles will "ping" with every shot. It's more noticeable with all-metal stocks. I had experienced this with a 1913 on a wooden stock. With a good cheek weld and conductive-through-cheekbone hearing, you could hear a ~1000 hz ping.

On this particular rifle two things were done that eliminated the ping. We tried one of the dampeners and it seemed to work. Shot success grew about 20% since the harmonic vibration had been damped. Then the dampening compound fell out of the assembly and I got rid of it. The ping was back.

Secondly, I later found that the 1913 is not a full free-float designed rifle. The stock is cut in a way that if it swells with humidity, it will touch the barrel halfway from the muzzle all the way back to the action. I had the stock milled out a bit and had a bedding job done on it. The resulting effort is on a YouTube video under my name. Zero shot ping after this operation! Scores have shot straight up, although I am still consistently shooting with strong Expert scores but not Master scores. Yet.

Now I have a special carbon fiber bloop tube and custom collar. Less weight than my earlier tube, but still not experiencing any ping effect. Scores continue to rise.

Eric U wrote:Theories abound about how tuning really works, but nobody has proven a thing. Maybe never will. Just go over to some of the rimfire benchrest forums and read the arguments either way to your hearts content.

All I know is it works and the why doesn't really concern me too much. I'm way more of a practical engineer than a theorist. Results on target matter to me much more than the number of dead snakes (integrals) written in a notebook.

I also don't think you are "damping" vibrations as much as adjusting them to get improved group results.

Eric U

Eric,
I have to agree that the truth in the rudiments of rifle tuning escape me as well. I have attempted to theorize on why my stock design works as well as it does and find some who will almost agree, to those that thinks it works for different reasons. In the end I really don't know.

I developed this stock in an attempt to forgo the batch ammo testing procedure and figured if I just purchased a high grade of ammunition I could "dial" it in . . .literally. I find that once I find the setting for a particular ammo I can shoot it for as long as the ammo lasts without re-adjusting the tuner. Changing brands and speeds of ammo does require a re-tune for example going from Wolf ME (1053) to Federal 922 (1088). Temperature or humidity don't seem to affect the tune.

I designed the stock for prone/3P shooting and gave myself an accuracy criteria of .250 CTC at 50 yards using a custom barrel which I though was sufficient for a reasonable shooter to score well with and possibly win. Although I have been successful in meeting my criteria I can't let it go and would really like to know exactly what is going on.

I think that the term "tuning the barrel" really needs to be modified to "tuning the shooting system" which most certainly includes the stock and accessories.

Jeffrey

Tuning the barrel has been the case for those that use the (muzzle type tuners). But either way..either at the end of the barrel or pressure on the action I suppose the effects coud be the same. We are changing the vibration frequency within the barrel. As theory goes we are finding the frequency of the vibration that has the least affect on the bullet as it exits the muzzle. If we are not changing the frequency, then we are just moving it so as to have the vibration at an optimal point as the bullet exits (as with pressure on the action). I have no experience with changing the pressure on the action, but we know good bedding clearly improves consistant results. Especially if your removing the action very often.

I've had similar results with my tuner as per different batches go. I don't have to re-tune unless I shoot something drastically different. As theory goes as well if the barrel is "tuned" no further changes are needed ever again. I don't know if that is true yet. I still tinker with it to see if I always come back to the same spot. I have all my data in writing to as to know what progress I make. Fun stuff!

To get really scientific look for the book "Science of Sound" a post grad textbook. My wife is an Audiologist and is very familiar with sound. Though she hates my sport she actually found this interesting and very plausible.

Tenring

There are videos on the WEB that show barrel vibration:

http://www.youtube.com/user/29426129#p/u/10/FaW_Hs0B79c

A couple of ideas/theories:

A barrel weight or "bloop tube" that extends in front of the barrel can hopefully move the node(s) to make the muzzle movement parallel and not angular. I believe this is impossible to achieve unless there's mass i front of the muzzle.

It could be a good idea to make the bullet leave the barrel at the extreme point of vibration (max amplitude) where the barrel is changing direction and thus not moving for a short instant (top or bottom of a sinewave). There may be several such points to choose from.

The vibrations are complex, there are several modes that one cannot expect to control simultaneously. You probably have to find "the worst offender", deal with that and ignore the lesser evils...

Eric U wrote: All I know is it works and the why doesn't really concern me too much. I'm way more of a practical engineer than a theorist. Results on target matter to me much more than the number of dead snakes (integrals) written in a notebook.

While I agree that it's results that matter...
I can't help but feel that if we understood the reasons WHY tuning works, we might be able to develop a process that would save ourselves a lot of time and ammunition in achieving the optimum settings for our rifle/ammunition combination - rather than having to go through the current "brute force", trial and error methods.

I also found this theory intriguing : http://www.the-long-family.com/OBT_paper.htm
It's from a Full Bore perspective but may be relevant to smallbore too. Not sure how adding weight to the muzzle would produce a tuning effect though.

I still would like to know how big an effect we're talking about. A full ring, 0.1 ring, or just what.

For all the thoughtful responses to my original post, many thanks!

pz

[quote="KennyB"]"On another site there is much discussion regarding bore curvature and how the orientation of the barrel effects the grouping ability of the rifle. The suggestion is that a slight downward curve to the bore produces the optimum results with minimum horizontal dispersion to the groups and vertical stringing due to varying velocity of the round being the dominant source of error."

One point I think that needs to be made clear is that while a downward curve is optimum if you have a curved barrel, the goal should still be to have a barrel with a straight bore, without any curve.

Walter

The rifle in the You Tube clip that Luftskytter posted seems to be an air rifle (note absence of muzzle gases). I find it fairly hard to believe that an air rifle has enough energy to set a barrel in motion that strongly. Any chance that what is being shown is really just the shooter's own motion?

It is wrong to think that this subject has not been researched and that it is mysterious and poorly understood. Good work has been done with instrumented guns, demonstrating the causes of barrel vibration and ways in which the problem can be attacked.

For various reasons to do with tradition and manufacturing convenience, rifles are generally designed so that induced barrel vibration is inevitable. One can therefore attack the problem by trying to reduce the vibration at source, by modifying rifle design, attenuating the vibrations by using lossy dampers, or reducing the effects of vibration on muzzle deflection at the time of projectile exit by altering the frequency of the vibrations with 'tuners'.

Ammunition testing isn't about finding intrinsically 'good' or 'bad' batches of target ammo - good ammo will be very consistent. What is needed is ammo that will cause the projectile to exit the muzzle at the most favourable time of the muzzle's deflection. Because the speed of sound in steel is so high, and the vibrations we are talking about are of fairly high frequency, tiny changes of (muzzle) velocity will result in the projectile reaching the muzzle at different phases of its vibration. So you can 'tune' your barrel to your ammo or you can 'tune' (by testing and selection) your ammo to your barrel. Yes, it's tedious and painstaking work.

Good, scientifically impeccable work has been done and published. Unfortunately, this authoritative information is swamped by the sort of usual half-truths, misinformation and other rubbish that clogs magazines and the 'net. The good stuff is out there (and at least two excellent books are widely advertised) - go and find it. Perhaps those in the know aren't too keen on pointing others in the right direction so that we may retain a competitive advantage...