Barrel Harmonics Question RE: Accuracy & Accessories.

[PegLeg45]

Everything in nature has its own 'frequency'. This frequency can be tuned, in rifles, by systems such ad the BOSS system and others. Some work by lengthening or shortening the barrel in small increments.

Induced electrical frequency is a little different than vibrational frequency in firearms, although they do overlap in the vibration department. I worked for years with electrical frequency to control the speed of machinery by regulating the hertz. Like changing the speed of a ceiling fan or dimming the lights through a dimmer switch, you are basically changing the speed (or cycle, hertz) of the flow of the electricity through the object. Standard flow of 120 volt appliances and such is 50 - 60 Hz. Take a potentiometer and drop that to 30 Hz and the item slows down (but the amperage increases and builds excess heat in the wiring if it is not heavy enough). Anyway, it is easy to get overly bogged down with this stuff, and it's been a few years since I've fooled with is. My old boss could explain it in better terms than I could because I'm not as good at getting some things out of my noggin and into written words.


The excerpt below explains harmonics as applied to firearms pretty well.

Stiffness

Stiffness of a barrel is proportional to the fourth power of the diameter, and inversely proportional to the third power of the length. Because of this, short, thick barrels will vibrate with high frequency and low amplitude, and long, thin barrels will vibrate with a low frequency and high amplitude. Due to the impact of length, barrel harmonics are primarily a concern with rifles. By using the shortest and/or fattest barrel possible, the amplitude of the vibrations can be minimized to the point that they are irrelevant to accuracy. Unlimited class benchrest shooting barrels, where weight is of very little consequence, have very large diameters; an outside diameter of 2 inches (5 cm) is not uncommon.

While standard rifle barrels taper from breech to muzzle, high precision rifles will often use a barrel with far less taper, called a heavy barrel, sometimes leaving the barrel cylindrical all the way to the muzzle, called a bull barrel. Either technique greatly increases the stiffness of the barrel by enlarging the average diameter, but this process adds significant weight as well. This can greatly increase the mass of the barrel, however; going from a lightweight sporter contour to a heavy barrel contour can double the mass, and a going to a bull barrel contour can more than triple it. Fluting, consisting of grooves machined in the outer surface of the barrel to remove material, can reduce the weight while maintaining most of the stiffness.

Barrel tensioning devices are a way of gaining stiffness with minimal increases in weight. They do this by using placing a lightweight sleeve, often made of aluminium or a carbon fiber composite, around the barrel, and then using a nut attached to the end of the barrel to tension the barrel and place the sleeve under compression. This serves to keep the muzzle closer to concentric and coaxial to the breech during vibration.

Harmonic tuning

The other solution is to work with the barrel's natural vibration, and tune the components so that the bullet exits the barrel as it is moving the slowest. The simplest approach to harmonic tuning is to concentrate on the ammunition. The internal ballistics of a given cartridge will determine its dwell time, or the time it takes from ignition to exiting the barrel. By experimentally matching the dwell time to the barrel's frequency, the best load for a particular firearm may be found. Similarly, handloading gives the shooter the opportunity to very precisely control the bullet velocity, and experimentally choose the optimum velocity.

If it is not possible or desirable to match the bullet to the barrel, there are a number of devices marketed to allow the barrel to be tuned to match the ammunition. There are a number of models of these that work in different ways. One type uses an adjustable damper or pressure bedding point to allow the shooter to find the "sweet spot", where it will do the most good at damping the vibrations that are affecting accuracy. Other tuners work by using an adjustable weight on the muzzle to alter the length of the resonant portion of the barrel, and allowing the frequency to be matched to the ammunition.

http://en.wikipedia.org/wiki/Accurizing

[tombogan03884]

Would a full length stock using barrel bands cancel out the vibration in a set up similar to this
http://www.ruger.com/Firearms/FAProdView?model=17143&return=Y

[Ping]

My shoulder hurts just looking at the photo of that Ruger.

[fightingquaker13]

My shoulder hurts just looking at the photo of that Ruger.

Its hell on the zombies though, particularly with the 458 Lott.
FQ13

[1911 Junkie]

Would a full length stock using barrel bands cancel out the vibration in a set up similar to this
http://www.ruger.com/Firearms/FAProdView?model=17143&return=Y

I wouldn't think this would be ideal. You are still connecting the barrel to the wood stock via the band and are, once again, going to be at the mercy of temperature and humidity making the wood swell and move, therefore, affecting accuracy.

[Sponsor]

[fightingquaker13]

I wouldn't think this would be ideal. You are still connecting the barrel to the wood stock via the band and are, once again, going to be at the mercy of temperature and humidity making the wood swell and move, therefore, affecting accuracy.

You're one hundred percent correct. That's why we free float barrels. I've never understood the European preference for this design, it only adds weight. However, in dangerous game calibers, like the ruger, the weight makes sense in terms of felt recoil. It also adds stiffness and protection to the barrel, the same reason it was standard on military rifles for generations.
FQ13
PS For dangerous game rifles like the Ruger, accuracy isn't the primary concern. What you want is good enough, and all the time. Its like an SD pistol. Is it enough gun? Can it hit the center of the brown, or spots or gray or whatever? Will it go bang no matter what?

[m25operator]

TW this is a great question and subject, and as you see a lot of variables come into play. I'll try to make some parallels that make sense.

1) Barrels swell and vibrate when a bullet is passing through it, .22 rimfire and up. Imagine the barrel as a diving board. Depending on the weight and speed of the diver jumping on it, it will flex a lot or a little. A stiffer diving board takes more energy to flex, this is why we like stiff barrels. If there were something under the diving board that it touched during the flex, that would influence it as well, a light load might not make it come into contact with the object, but a heavier one would. This is why we like free floating barrels. If you put something under the diving board that touches it from the beginning, it is going to influence it a lot, change the characteristics of the board completely. Like a bipod on the barrel or resting the barrel on anything, sand bag, fencepost etc...

2) Frequency and amplititude. And why it matters.

Frequency is how many times something happens over a specific amount of time, amplitude is how far it moves in this context. Take the diving board example again, you flex the board and it goes up and down during and after the load has been delivered, it will continue to go up and down until is settles. How many times it went from center, to up and down, is the frequency, how far it moved, up and down is the amplitude.

Now imagine your barrel as the diving board, as the bullet go's through the barrel, the end of the barrel is beginning to whip, depending on the frequency, the bullet may exit one time on the up cycle and the next on the down cycle, there fore the bullet will vertical string the shots. Depending on the amplitude, the distance the muzzle moves during the cycle and when the bullet exits, can make more distance between the shots on paper.

Skinnier barrels move more, thicker barrels move less.
Thicker barrels are more resistant to movement from outside sources, like touching the stock or a bipod, or resting on a fence post, but it still makes a difference. Just less of one.

Fine tuning your loads, can reduce the frequency and amplitude, the idea is to have bullet exit as close to the beginning static position of the muzzle as possible, therefore  less dispersion.

Out side influences like the stock touching the sides of the barrel or loose stock screws cause the horizontal or all over the place groups, especially as the barrel gets hot, cold metal is stiffer than hot metal, not uncommon for a skinny barrel hunting rifle to shoot the 1st 3 shots very well, and then open up quite a bit.

Hope I satisfied ya - HAZ

[Rastus]

............
Hope I satisfied ya - HAZ

He deserved that.....   

[DesertMarine]

Several years ago, Precison Shooting magazine did a series of articles on barrel dampening.  Basic conclusions as I remember them, is that yes, dampers do affect accuracy.  Vibration waves were checked, like waves in oscilloscope waves, and the authors concluded that where you put the damper will make a difference.  Also where you cut a barrel will make a difference.  One guy talked about rimfire rifles where he preferred barrels longer than 20" and slimmer rather than 20" or shorter but thicker.  It was interesting reading but did get confusing in some parts.  Isn't Tatical Solutions marketing a barrel damper that they claim is pretty good.  Winchester and Browning had their version, "The BOSS" on their rifles for a while. 

[tombogan03884]

I wouldn't think this would be ideal. You are still connecting the barrel to the wood stock via the band and are, once again, going to be at the mercy of temperature and humidity making the wood swell and move, therefore, affecting accuracy.

I'm aware of that, but my question, like the thread, pertained specifically to the harmonics question.

You guys are wimpy, click on the specs, the pictured rifle is a .308
My shoulder hurts just looking at the photo of that Ruger.