Is a barrel's resonant frequency static?

[shortpants]

I like it, simple and accurate! I'm still figuring out what works best for me. I'll have to keep your methods in mind. I use a chrony as just another tool. It gives me a general idea where I'm at but mostly I just use it to keep track of my ES.

 

[Trickymissfit]

Here's two barrels fit to receivers bolted in a stock; both 30 caliber. The hole diameter of .306 inch was used to equal what a typical 4-groove area of .308" groove diameter would be (.0736 square inch). My software only uses one diameter for the hole in the barrel. Use the profile below for dimensions listed:

First barrel, typical 24" sporter rifle:

hole diameter = 0.306 inch
breech diameter A= 1.25 inch
reinforce large diameter B= 1.25 inch
reinforce small diameter C= 0.8 inch
muzzle diameter D= 0.6 inch
reinforce length AB= 2.75 inch
reinforce taper length BC= 3.25 inch
main taper length CD= 18 inch
total_length= 24 inch
total_weight= 3.191 lb

Second barrel, typical 30" Palma rifle:
hole diameter = 0.306 inch
breech diameter A= 1.25 inch
reinforce large diameter B= 1.25 inch
reinforce small diameter C= 0.92 inch
muzzle diameter D= 0.82 inch
reinforce length AB= 2.5 inch
reinforce taper length BC= 2.5 inch
main taper length CD= 25 inch
total_length= 30 inch
total_weight= 5.095 lb

Which one's stiffer and by how much?

One of them vibrates at about 71.5 cycles per second and that's the frequency where the greatest amount of bore axis angular spread happens at the muzzle as it whips in the vertical axis for that barrel's contour. It goes through one cycle in about 14 milliseconds. Bullets take a little over 1 millisecond to go from case mouth to the muzzle.

PS: I might be persuaded to calculate then compare two other barrels, but only for nice guys.

just to add to this a thing that Dan Lilja stated a few years back. With typical #7 barrel contours at 26" and 21" the shorter barrel is 2.75 times stiffer. Was it Kelby or Borden that called very long stainless steel barrels "tuning forks"?
gary

 

[Trickymissfit]

Yes. And bazookas, paint ball guns as well as a 66 foot long 120 ton 16 inch gun barrel from a battleship. Dry firing a hunting rifle makes its barrel vibrate a tiny bit. If you take the barrel out of the receiver then smack it with a hammer, it'll vibrate at a frequency several times higher than when its breech end is fixed in a receiver bolted into a stock.

a really good example of barrel vibration that is also easy to see, is to watch film footage of an M107 SPG shooting near max range with a zone three powder charge (seriously overbore!!!) You will see the barrel wiggle all over the place (probably two or three inches in each direction.

gary

 

[Trickymissfit]

That weight on the barrel's front won't stop it from vibrating. It only changes the frequency the barrel vibrates the most at. It doesn't balance any harmonics, but that's a nice name for it.

Its just someone putting a name on something that he didn't have a name for and used a word that made sense to him to describe something he understands but didn't have a name for it.

Wow that didn't make any sense either. Glad you were able to make sense of it and describe what is happening. If left to me I would of had to say "Don't listen to what I am saying, listen to what I am thinking"

I spent a lot of time reading Bill Calfey's writings and he always claimed he could stop the barrel and that it wouldn't change from load to load. He also never did really show anyone how to do it just said he could. Many tried it but to my knowledge they only succeeded in changing the harmonics of the barrel similar to what browning does with the boss. Thats not stopping it.
Anyway thanks Bart for the good explanations.

Bill thoughts have been proven out over and over. Try winning a match at Borden Indiana without a tuner!
gary

 

[Bart B]

I spent a lot of time reading Bill Calfey's writings and he always claimed he could stop the barrel and that it wouldn't change from load to load. He also never did really show anyone how to do it just said he could. Many tried it but to my knowledge they only succeeded in changing the harmonics of the barrel similar to what browning does with the boss.

That's what Bill Calfee does; but it's the resonant frequency of the barrel that changes. It's exactly the same as what Browning does. Harmonics are even multiples of that frequency. And harmonics cause the least amount of angular motion in the barrel when they're whipping. Those weights can be placed anywhere along the barrel and that'll change the point in space the muzzle points when the bullet exits. Those tuners have been reasonably popular in .22 rimfire competition but seldom seen in centerfire disciplines save for a few benchrest matches. As centerfire bullets spend about 1/3 the time in the bore as rimfire ones do, the slower ones are easier to time/tune the whip so they leave at the best moment of muzzle movement.

A given barrel whips or vibrates at the same frequency regardless of the load shot. It's a fundamental property of the barrel, not the ammo. The rigid (or flexible) properties of the barrel stay the same.

 

[Bart B]

Here's the answer; I won't keep some readers waiting.......

First barrel, typical 24" sporter rifle:

hole diameter = 0.306 inch
breech diameter A= 1.25 inch
reinforce large diameter B= 1.25 inch
reinforce small diameter C= 0.8 inch
muzzle diameter D= 0.6 inch
reinforce length AB= 2.75 inch
reinforce taper length BC= 3.25 inch
main taper length CD= 18 inch
total_length= 24 inch
total_weight= 3.191 lb

Second barrel, typical 30" Palma rifle:
hole diameter = 0.306 inch
breech diameter A= 1.25 inch
reinforce large diameter B= 1.25 inch
reinforce small diameter C= 0.92 inch
muzzle diameter D= 0.82 inch
reinforce length AB= 2.5 inch
reinforce taper length BC= 2.5 inch
main taper length CD= 25 inch
total_length= 30 inch
total_weight= 5.095 lb

Which one's stiffer and by how much?

The first one, 24 inch sporter stiffer with a resonant frequency of 71.55 Hz.

The 30 inch Palma barrel's resonant frequency is 38.88 Hz

For comparison, a 28 inch 30 caliber magnum 2-taper barrel typical of what's used in NRA long range matches:

groove diameter H= 0.306 inch
breech diameter A= 1.2 inch
reinforce large diameter B= 1.2 inch
muzzle diameter C= 0.9 inch
reinforce length AB= 3 inch
reinforce taper length BC= 25 inch
total_length= 28 inch
total weight = 6.522 lb

fundamental frequency = 52.11 Hz

 

[Trickymissfit]

That's what Bill Calfee does; but it's the resonant frequency of the barrel that changes. It's exactly the same as what Browning does. Harmonics are even multiples of that frequency. And harmonics cause the least amount of angular motion in the barrel when they're whipping. Those weights can be placed anywhere along the barrel and that'll change the point in space the muzzle points when the bullet exits. Those tuners have been reasonably popular in .22 rimfire competition but seldom seen in centerfire disciplines save for a few benchrest matches. As centerfire bullets spend about 1/3 the time in the bore as rimfire ones do, the slower ones are easier to time/tune the whip so they leave at the best moment of muzzle movement.

A given barrel whips or vibrates at the same frequency regardless of the load shot. It's a fundamental property of the barrel, not the ammo. The rigid (or flexible) properties of the barrel stay the same.

refering to Bill Calfee's at the time theory. He wrote several articles in P.S. on the subject and I remember reading them because he was just begining to touch on vibration and what it does. I was doing a little bit of work on the stuff in another field (a sin wave is a sin wave no matter where it is). I honestly didn't grasp everything after reading his stuff a couple times, but was interesting anyway. A few weeks later I went to a seminar at Brookville IN, and I brought his articles, and the instructers were somewhat interested. After they got done explaining to me what he was trying to achive, I kinda got the hang of it. While this was going on Tony was also reading and doing his own tests ( unknown to me at the time). Tony came to me and asked me to build him two or three threaded clevises to hang rifle barrels on, and that's how I found out he was interested in the subject as well. We hashed it over for two or three months, and then he has an idea on what to do, and that's where I started to design one. The tuner was redesigned about a half dozen times before we were ready to cut the first piece, and even then I was very leary about it. The last obstical was how to attach it to the barrel without messing up the bore. I wasn't the slightest bit happy with the method we used, and I'm also certain the group size opened up because of this alone. The first couple we built had a little more adjustment than really was needed, but we also were unsure of our ability to step into this new form of "black arts". I remember that the mounting thread was somekinda super oddball one that came from taking the minimum off the barrel's O.D., and then threading it. I would never have done that again, but it still worked pretty well. My second generation mount was much better.

The first ones we built had about 1.25" of adjustment, and we both felt later that half that was more than enough. We also found that it was near impossible to get the threads tight enough, but we fixed that issue fairly easy once we got to thinking. What I found most interesting after he went thru the process (and logging down all his numbers) was that the brands of ammo only varied about two turns on the tuner. He could make ammo that just would group come right in to where they were about one third the group size prior. I did go ahead and built three or four of them to fit a couple 541's, but never got the time to mount them (541's had other major issues to tackle!). Plus just about that sametime I came into another way to sort of help to deaden that sin wave.
gary