A little research results in several "theories" w/regard to barrel vibration nodes. Could someone who actually knows, please straighten this poor southern boy out?
What I see:
I think everyone agrees that a short/thick barrel has less muzzle moment overall than a long/thin barrel.
I read that barrel vibration originates from a bowing barrel.
(thin barrel, more bow -larger waves, -freq/node width?)
I read that barrel vibration originates from rifling.
(Fast rifling/hi velocity -higher freq, wave amplitude?)
I read that barrel vibration originates from pressure peak.
(like an engine "ping", hi Pk, -hi amplitude, -freq/node width?)
Then, more abstract, firing pin, action and any connected metal anomalies.
What I lack:
The actual cause of the major vibration waves.
Are they resonant?
The frequency of these waves, and following nodes.
Thick barrel=hi-freq/narrow node, or lo-freq/wide node?
How parameter changes affect node frequency.
Rifling, powder speed, barrel time, peak pressure amplitude, barrel length, barrel stiffness?
I need help here.
Just stepping back and looking at it; It would seem that if I strike a short or thick barrel at the action with a hard plastic mallet, I should expect a higher frequency/lower amplitude wave, as measured with hall effect sensors adjacent to the muzzle, compared to the same test with a long or thin barrel.
To me this would suggest that stiff barrels would deliver lower overall dispersion throughout a range of loads than a whippy barrel.
But, its higher frequency(the double edge) would cause it to be difficult to keep within it's relatively narrow nodes.
What do you think? Do stiffer barrels (at the magic load) need lower ES to shoot consistant? Is a whippy barrel (at it's magic load) more tolerant to higher ES?
A couple comments here: You are correct about a longer bbl "moving" more than a short one, but when it comes to bbl harmonics, what is happening is that as the bullet travels down the bbl, the majority of the movement is in a circular motion due to the spin the rifling is putting on the bullet. These harmonics cause the bbl to move in a circular position at the end and not so much in a "whipping" motion. Due to harmonics, the bullet travels down the bbl and exits at any given point in this rotational movement. Thus, you might have a bullet leaving the bbl as it is passing 12:00, then the next shot at 3:00, and the next at 6:00. This is the cause for triangular patterns on the target. I have found that by putting a small piece of delrin (which has a very low heat expansion factor) in between my stock and the end of the bbl, I can "tune" the harmonics of the bbl for any given load. Also, it depends on other factors such as the composition of the bbl, the surface area of the bbl, and the weight. I do have some printouts using an O-Scope to measure vibration frequency along with all the math showing the Sin waves of one of my bbl's and I think it may have a lot of the info you are looking for. One thing I can recall about V Node Q is that; if the forces acting upon an object vary quickly, as in a rifle bbl, then, the displacement within the node is going to vary relative to time. This being true, then the outside forces are extended by the distribution of the forces of interia and, if I am not mistaken (which I may be), these are directly proportional to the acceleration. Again, if I am not mistaken, you can use the equation: Ku = Q - Mu". Where: M = diagonal mass of bbl, u" = the second derivative of the Q node displacement or rotation. It's been a LONG time since I took Engineering Physics, so that's mostly all I remember. When Q starts to approach 0, you run into Eigen Values and it gets difficult. If I can find all the stuff I did on the oscilloscope I will forward it along to you. I know I didn't answer all your questions, but, hopefully, this can be of some help to you. Good luck!
The reason I ask is, during ladder testing I've seen almost no horizontal component in the the patterns. This really does fit what Harold had found, and as I understand what you are saying here, it is the opposite, which I've never seen.
Removing the error introduced by the increasing MV while testing at 300 yards, the nodes still have had just vertical shifts, never horizontal?
To some extent, I guess that would make sense to see more vert shift than horizontal if the rifle, ideally, is going to be moving in a straight line backwards during the recoil, but, it seems as though there should be at least some hor. shift. I haven't read that book yet, but I am very interested in reading it though. What is the name? I will see if I can find my printouts and, also, do a little more research tonight as I would probably benefit myself as well to brush up! Let me know what you find and, if I find anything else of use to you, I will forward it along.
During ladder load testing, I have had some loads show some diagonal shifts and some make slight shifts left or right but never true diagonal continous movement as the load increased. I have however had some of the loads move up and then shift down again (as load increased) but it was primarily verticle and as the load continued to climb, the pattern would move back up again.
Could it be that barrel bow causes the wave? Like a bourdon tube straightening w/pressure. And that, rifling(bullet twist acceleration/torque) simply induces rotation into a vertical motion, which is already traveling toward the muzzle? Pushed by the bullet.
I realize a component of the muzzle motion is circular. See it in ladder load testing.
I'm sure the barrel is trying to unwind as long as the bullet is accelerating against the twist. Probably lengthens the barrel a bit, for a moment.
But, Would a barrel w/zero twist have no vibration/nodes?
This is hardest for me to understand. Surely the vertical motion would remain. Perhaps after the bullet has left.
I mean, I don't know. Need a reason that other theories cannot be the cause, as well as support FOR a given cause.
If that make sense
I have not always been able to get high enough to see the shift back down when increasing the charge, as I often hit max load before the printing downward trend comes on before it goes up again. I have had loads that never went down, and just hovered then went on up again. I've also never found a load yet that had more than two sweet spots, and most often it's just one if I don't work a wide charge range and just stay near the top few grains.
You're over my head, you and Austin can take over here, I'll set back and listen. [img]images/icons/grin.gif[/img] [img]images/icons/wink.gif[/img]