Airgun barrel harmonics?

[Poor-man Plinker]

It is sad when the most comprehensive info you can find on-line is in Wikipedia. However:

"rapid increase in pressure causes the barrel to vibrate at a certain natural frequency, much like a tuning fork. The point in time at which the bullet exits the barrel will determine the orientation of the muzzle relative to its rest position. Exiting near a peak or valley in the motion means the muzzle is relatively stationary, and shot dispersion will be minimized; exiting between a peak and valley means the muzzle is rapidly moving, and shot dispersion will be greater. There are two ways to address harmonics; reducing the amplitude with a stiffer barrel, or working with the natural frequency to minimize dispersion. 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."

So we can change pellets to change weight, skirt diameter, etc. and the resulting "timing." Just hoping like heck we find one that leaves the barrel in a peak or valley of the barrel "whip." Or, there are barrel shrouds, muzzle weights (no matter what name you give them) and vinyl tube overlays that have been recommended without any before-and-after group sizes posted in what I saw.

I guess I could add or remove more barrel clamps in the 2240 design, or, move them around on the gas tube to vary the "whip" until I find a sweet spot. Perhaps you are waiting for a question from me ha ha ha....

OK: Would not a free floating barrel just change the harmonic as opposed to a "bedded" or clamped barrel? It seems a clamped barrel would actually shorten the whip effect... Why would one be preferable to the other?

And, B: What has anyone done to affect harmonics that they could report here on the forum?

[rsterne]

I tried a series of 7 muzzle weights on a fully floated 24" Crosman barrel.... They varied from 7 grams (yes, only 1/4 oz.) to 37 grams by only 5 gram increments.... In the one complete trial I did, with the pellet which showed the best accuracy without weights, (JSB Express) and shooting at only 20 yards, you could see the group close up as I added weight, and then open up again.... In addition, the heaviest two weights caused the POI to be lower on the target, the heaviest by nearly an inch at 20 yards, with some noticeable (nearly) vertical stringing.... My theory was that the best weight (#4 out of 7 = 22 grams) had the pellets leaving the barrel at the top of an arc (ie a "peak), causing a tight group.... while the heaviest had them leaving during the downswing (but before reaching a "valley"), hence the lower POI and vertical stringing.... The pure weight likely was bending the barrel downwards as well, contributing to the lower POI.... This could have been confirmed by using even heavier weights to see if the group tightened up again at a lower POI, but I never bothered....

It would have been great it that was the end of the story, wouldn't it?.... Unfortunately, if I changed the velocity, I could make the barrel "prefer" a different weight.... With a slight decrease in velocity (~20 fps?) I could make the gun prefer no muzzle weight at all.... In addition, I could improve the groups with a couple of pellets which shot "well" but not the best in that barrel.... to the point that the best groups with a "good" pellet were now better than the worst groups with my "best" pellet.... This shows promise that if you have a pellet you really like that shoots "OK" you may be able to tune it to shoot great by playing with barrel weights and/or velocity....

Bob

[sholo]

Great information as usual, Bob. Interesting too...

I thought I read in a thread somewhere awhile back (maybe by Sean, IIRC?) about "sliding" or "adjustable position" weights that you could move along the barrel to find the sweet spot as well. Ever have any experience with them, or am I just imagining things again?

Seems like a fellow could spend a couple of afternoon's and a tin or two of pellets playing around with this idea!

.....

Todd

[SeanMP]

Okay I have to answer this one bottom to top.

B) It would be simpler to list what I have not tried

Clamping the barrel creates a node or multiple nodes. The barrel is going to vibrate at it's resonant frequency no matter what. If you clamp it at X you can have one of two things occur a) you create two segments that have been clipped at exactly the same point in the sinusoidal wave and everything is peachy. Basically you have two pieces wiggling away, each a perfect mirror of the other... or ...more likely b) you have two segments clipped at different points and when the wave energy comes to the boundary point and reverses the two interact creating both a harmonic and a chaotic wave.....darn tough to tune those chaotic waves as they take a long time to do the same thing twice.

The more points of contact, the more nodes, the more likely it is you'll create a vibration that is purely chaotic. Even if you park a truck on the barrel it's still vibrating, all you have done is altered the amplitude not the frequency. And that is of course transferred to the pellet and it leaves with that vibration. So it's better to tune it so that the pellet is leaving at the instant that the system is effectively "still". Don't get me wrong controlling the amplitude is still a good idea because it lessons the rate of change.

[SeanMP]

Great information as usual, Bob. Interesting too...

I thought I read in a thread somewhere awhile back (maybe by Sean, IIRC?) about "sliding" or "adjustable position" weights that you could move along the barrel to find the sweet spot as well. Ever have any experience with them, or am I just imagining things again?

Seems like a fellow could spend a couple of afternoon's and a tin or two of pellets playing around with this idea!

.....

Todd

Ya that was me Todd. Having a weight that you can slide around is the current in vogue way of doing things. It allows the use of a smaller amount of weight which is then moved to the sweet spot through testing. If everything is right you can end up with a tune that will be effective across a range of velocities. The 50m prone and 3P folks spend a lot of time tuning this way because even spendy match ammo has a range of velocity.
One of the more common tuners out there for Olympic level folks is called the Upta Tuner

[Gippeto]

Not sure who has seen this before....

http://www.varmintal.com/amode.htm

http://www.varmintal.com/atune.htm

Al

[rsterne]

I've seen those animations before.... it's pretty amazing we can hit anything.... *grin*....

Bob