Harmonic Barrel Tuners - Any Experience?

[Slavia]

There's definitely movement there:

[Kailua]

Gerard mentioned using a vise and high speed camera for testing harmonic movement.  Would a precision dial indicator with a stand based at the breech work?

[Gerard]

If you can see the changes fast enough a dial indicator sounds great. Perhaps an indicator which stops at high points, not springing back? I don't know if they make those...

[Kailua]

Gerard you had mentioned a high speed camera.  Which might show the back and forth movement of barrel harmonics with a dial indicator.  Anyways just thinking out loud.

[rsterne]

There is no questions that barrels vibrate when the gun is fired.... There are two approaches to this....

1. You can try and minimize the amount of vibration (amplitude) by stiffening the barrel.... The normal methods are a larger diameter (either a larger barrel per se, or a carbon fibre sleeve).... or putting the barrel in tension (which raises the frequency and lowers the amplitude, like tightening a guitar string).... or

2. You can try and alter the timing of the muzzle movement, so that the bullet arrives at the muzzle when the barrel is pausing at the end of a swing, and preferably so that it is at the top when the slowest bullet arrives there.... This launches the slowest bullet on the highest arc, helping them to converge at the target.... This is usually done by movable weights....

You can, of course, use both approaches together....

Bob

[Taso1000]

Bob,

I was trying to figure out a temporary weight to clamp to the barrel for testing that doesn't require machining.  I know there are those LimbSaver devices but I thought of hose stops.

They won't work on a B51 as is because the barrel is so short, unless I use one half and figure out a bar between the barrel and air tube to secure it.  But they may work for others and shouldn't be very expensive.  I figure if you find the right spot and weight, you can make something more aesthetically pleasing.

I haven't figured out anything economical to use on the end of the barrel.   

Taso

[tehsquirrel]

Sorry to have been less than clear. I've not personally tried hitting barrels to test harmonics/vibration. I was only suggesting clamping a barrel and hitting it (with whatever you have handy to make it ring, such that you get it moving but don't damage the finish) and listening to it ring as perhaps being a useful component in getting a rough idea how the barrel is behaving. A barrel is a metal rod. Sure it has a hole in it, but so what? It's a rod which has particular physical characteristics as regards modulus of elasticity and such, controlling the amplitude and frequency of vibrations particular to its density, length, OD, bore size, any tapering which might be present, muzzle device mass, etc. It's behaving exactly like a tuning fork when struck. It's moving, thanks to a violent impulse. And of course that impulse is different from hitting it from the side, but I'm talking about finding a good starting point from which more refined, delicate variations in mass and/or placement of a weight can then proceed. Otherwise you'd just be taking a wild guess (or W.A.G.) as to how much metal and where to put it.

Clamping it securely at the exact same exit point from the clamping caul (and manner if possible - a pair of semi-cylindrical cauls to enclose the barrel in a vise would be ideal, or just clamp the rifle barrel block itself with barrel mounted if that's practical) as it exits the mount on the rifle simulates the same freedom of vibration as when mounted in the gun, or close enough for getting a ballpark guess going on vibrations. Adding some form of weight at various points and striking the barrel to get it vibrating, one should be able to hear the biggest, sloppiest vibration of the barrel without the weight, then find the placement and amount of weight which most effectively dulls that vibration. Most any material has a natural frequency of vibration. Finding a counterweight which is optimal for interfering with that vibration is likely going to get you close to stopping the barrel's movement during a shot cycle.

Getting into nodal points and peaks of vibration would be great if you had the proper lighting setup and a high speed camera. Analysis of the actual motion of the barrel would greatly enhance your ability to quickly establish the effectiveness of various weights and placements. But of course most of us don't have access to this equipment nor the knowledge of how to use it. So I suggested using your ears to determine how much vibration your weight is damping, finding a minimum value by a bit of trial and error in the vise, then further refining this by shooting groups and making small adjustments to the mass and location of that mass on the barrel.

I was thinking along similar lines and feel like this thread could produce or provoke further study to find some standard for identifying and calculating barrel harmonics.

I thought of two ways to record the data:

1)  using a detailed high speed camera to record barrel movement

2)  using a microphone or parts of a microphone (imagine like 20 mic's arranged evenly along a barrel) to record frequency along the barrel


I guess after being in a position to record the harmonics then you are in a position to see possibly which harmonics have negative or positive effects on shot placement.  My guess is that relevant frequencies are going to come into play here (related to  time traveling through the barrel, pellet size, etc).  Introducing harmonics through artificial means may provide more insight (such as with loudspeakers or tuning forks). 

The 3rd step would be identifying how to eliminate unwanted harmonics and how to possible increase any harmonics that could possibly increase accuracy.


The reason I checked out this thread was because on my xisico XS25 when I took off the front factory sight I noticed that the groups opened up something awful.  Putting it back on fixed the problem, and I've had an eye out for that type of stuff since then.

[Gerard]

Indeed it's a fascinating subject, though the mechanics and even the gross concepts involved in starting any method of investigation are somewhat daunting to someone like me who is not an acoustical physicist. I've studied some of that stuff, but mostly pertaining to my own field of work and mostly not applicable to barrel vibration. That said, I doubt that a series of frequency recording microphones placed along a barrel would offer any relevant insight, as the fundamental vibration is really the only one of interest. The barrel's 'natural frequency' will bring the largest amplitude of oscillation by far, with overtones/harmonics playing much smaller roles.

Even isolating the particular frequency of the fundamental tone in itself seems unlikely to be of much help. What we'd be after is rather the elimination of that oscillation, not re-tuning it to a different pitch. More like what a violinist does when they slip a rubber mute onto the bridge - dropping overall output volume dramatically through direct damping of bridge movement with an appropriate weight of rubber (or in the past ebony, or even steel with a lead weight for 'practice mutes' which are meant to prevent disturbing the neighbours. Modern practice mutes are usually a dense rubber as the inherent vibration absorbing characteristics are more effective than just adding metal mass.

I suspect that the ideal barrel whip damping device will not be just a finely adjustable weight, but will also have some of the flexibility of rubber built into its mounting. Perhaps a really soft layer something like sorbothane shoe inserts, in a sleeve between an adjustable metal weight and the barrel? This would offer both a mechanical dead blow hammer-like effect of the heavy metal part to cancel out the fundamental, while also damping any overtones which have less energy by means of the squishy rubbery layer.

[Aged]

Hi Bob,
I just read your year old post now and was wondering if you went through with that project.
If so was it worth it because I'm thinking of trying that myself.
Ed.

[Doug Wall]

I haven't tried solid mass harmonic tuners, but I have used the LimbSaver Barrel De-resonators. These "rubber" dampers slip over the barrel and seem to damp out vibrations. I have a couple on springers (Benji Trail AW NP, XS-25) and they seem to work for me. I don't have any before/after group pictures, but they certainly help tighten up groups For about $10 from Ebay, it's a pretty cheap experiment. There are two sizes- regular, and bull barrel model. If you have a large shroud, you'll need the bull model.

[Aged]

Thanks Doug but I want to use it on my pcp and the air tube runs the full length of the barrel. Only the air stripper extends beyond the tube so there's no room for it.

[K.O.]

Me I think the first thing to do is to thimble the barrel... then fine tune or damp from there...

[Aged]

Kirby I don't know what you mean by "thimble the barrel".

[Aged]

If it means to stiffen the barrel with a sleeve I have a carbon fiber one on it.

[K.O.]

http://benchrest.com/showthread.php?96044-Barrel-Indexing&highlight=indexing

dang photo buck...et

http://www.network54.com/Forum/79537/thread/1368912056/Marauder+build+is+finally+done

[Aged]

Thanks Kirby, that was an interesting read. From what I gathered the 'thimble' is used so the barrel can be rotated (indexed) while it's port stays aligned with the transfer port. Vertical indexing sounds like a good idea since I agree no barrel is or could be made perfectly straight. I see it as particularly useful in FT and hunting where if the barrel curved to one side the windage turret would also have to be adjusted for every new distance. I will definitely look into getting my other guns fitted with a thimble so I can vertically index them.
Indexing the gun I'm preparing for the EBR competition wouldn't be practical now though because or the way I had to secure the CF sleeve to the receiver. Also it
wouldn't help as much since I'll only have two distances to scope in for.