Is a barrel's resonant frequency static?

[SidecarFlip]

Interesting Bart.

I shoot a .223 in 100 yard comp and it has an adjustable harmonic damper that I actually mark (the barrel) as to what loads do what so I can index the damper according to load and bullet weight. The 'sweet spot' varies with round chambered and it don't change from cold bore to warm.

People ask me what all the paint marks are for all the time.....

 

[Bart B]

I shoot a .223 in 100 yard comp and it has an adjustable harmonic damper that I actually mark (the barrel) as to what loads do what so I can index the damper according to load and bullet weight. The 'sweet spot' varies with round chambered and it don't change from cold bore to warm.

The reason that works is you're timing the barrel such that it's at an optimum point on the muzzle axis' up swing to be at a higher angle for lower velocity shots that leave later and at a lower angle for high velocity shots that leave sooner. It compensates for bullet drop with an elevation correction.

The Brits learned this in the early 1900's with their .303 SMLE's using ammo burning cordite. The muzzle velocity spread was so great and their very whippy barrels in those rear locking actions were compensated for at long range quite well. They had more vertical shot stringing at 300 to 500 yards than at 600 to 1000 yards with their best ammo used in fullbore (long range prone) matches. They used the same ammo in Mauser 98 actions to shoot better scores at 300 to 500 yards as their barrels didn't whip as much.

 

[SidecarFlip]

I know it shakes (vibrates) because it's a constant chore setting the setscrews that loosen up. They tend to get loose after 10 shots or so, so I keep a T-handled hex socket wrench on the bench at all times. Threadlocker don't work in this application. I considered going to a nylon patched (shakeptoof) setscrew but it's not a big deal checking tightness.

I do appreciate your insightful posts. I understood the principle behind the damper but not why it works...until now.

I've never used one of those rubber things (doughnut) that fits on a barrel. Looks to me like it's not worthwhile, but maybe it is.

 

[Bart B]

A few years ago on Benchrest Central Bill Calby who is a small bore gunsmith made the statement that he could stop a barrel from doing just that by the use of a what I call a harmonic balancer on the end of a barrel.

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.

 

[Bart B]

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.

 

[SidecarFlip]

So, basically, the premise is..... If it goes bang, it vibrates.....

Does it apply to airguns and air rifles? You still have a projectile accelerating in a bore....?

Just air pushing. No bang, whoooooosh.....

 

[Bart B]

Does it apply to airguns and air rifles? You still have a projectile accelerating in a bore....?

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.

 

[BillR]

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.

 

[shortpants]

Bart thanks for your informative posts!

Will 2 barrels of the exact same specs (length, contour, caliber, weight, etc.) vibrate at the same frequency? If so would it be possible to know what velocity to load for a barrel without doing ladder or charge weight testing? Obviously you would still have to do load work up to get to the (known) velocity and check for pressure but is it possible for a barrel maker to give a velocity range that would work best with a particular barrel? Seems far fetched but you guys amaze me with your knowlege all the time!

 

[Bart B]

Bart thanks for your informative posts!

Will 2 barrels of the exact same specs (length, contour, caliber, weight, etc.) vibrate at the same frequency?

Yes. Why not? They're both identical in material and shape.

If so would it be possible to know what velocity to load for a barrel without doing ladder or charge weight testing? Obviously you would still have to do load work up to get to the (known) velocity and check for pressure but is it possible for a barrel maker to give a velocity range that would work best with a particular barrel?

No, it's not possible to know what velocity to load for. Two barrels with the exact same dimensions for chamber, bore and grooves plus outside profile will not shoot the same ammo at the exact same velocity. While it's possible for a barrel maker to give a recommended velocity but not a good idea. The barrel maker does not know what cartridge nor profile his barrel will end up for. I've never been concerned about the velocity my match ammo shot bullets at; I rarely chronographed them. As long as they all land close together on target, how fast they get there ain't important. I"ve worked up only one load, but otherwise used the same ones folks shooting top scores use; it's worked for several barrels chambered for different cartridges and bullet weights.

I've never believed "ladder testing" was meaningful. First off, it's based on the premise that best accuracy happens when the bullet leaves the barrel when the muzzle is at the top of its movement. But nobody's ever done a test with the right stuff to measure when in a barrel's whip cycle does the barrel leave. As long as they all leave at about the same place, it doesn't matter. I've cornered the perpertrator of "Optimum Charge Weight" on this and he's never explained his way out of the corner. It's just a theory.

Second, shooting one shot with each load in a ladder test is statistically inaccurate. If one uses ballistic software to calculate bullet drop at long range for half-grain increments of charge weight, they'll learn they need a 1/4 MOA rifle with all charge weights for the drop data to be meaningful at longer ranges.

Third, people have shot the same ammo in short skinny barrels and long thick ones and got the same 1/2 MOA accuracy at long range. Those barrels whip at different frequencies and bullets leave over a wide range of barrel whip. Some of us did a test years ago with a really good lot of 7.62 NATO match ammo in M14NM and M1 service rifles as well as a Model 70 match rifle. Three different barrel lengths, 22, 24 and 26 inches, three different muzzle velocities and three different barrel profiles; skinny, medium and heavy. They all shot about 6 inches at 600 yards; as good as the best arsenal match ammo's ever produced.

 

[shortpants]

Makes since. I did not think that question all the way through before posting it. Different powders, cases, primers, bullets, how could a barrel maker suggest a velocity with so many variables.

I understand completely what your saying about shoot for groups and who cares about velocity but for the long range hunter minimum velocities needed for proper expansion come into play. You have to know where your starting from to know where your going and to be able to utilize a ballistic calculator. How do you go about this without a chrony?

 

[Bart B]

I understand completely what your saying about shoot for groups and who cares about velocity but for the long range hunter minimum velocities needed for proper expansion come into play. You have to know where your starting from to know where your going and to be able to utilize a ballistic calculator. How do you go about this without a chrony?

I use Sierra Bullets' ballsitic software to see how fast a bullet has to leave at to keep it supersonic all the way down range. Then use powder charges that produce velocities about 100 fps higher than what I think the bullets leave at; close enough for me.

I'll sight in a new barrel at 100 yards, then set the sights for a zero such that bullet impact is below the aiming point equal to bullet drop plus sight hight above the bore. Ballistic software then tells me where to set the sights for different ranges within 2 or 3 MOA out to 1000 yards. Actual shooting refines the sight settings for each range. 30 caliber magnums' bullet drop at 1000 yards has a vertical spread of about 30 inches for a 100 fps spread in muzzle velocity. For a .308 Win., it's about 40 inches.

Using the same sight settings as a previous barrel had for a given cartridge have typically been within 1 MOA through 1000 yards. But most of my 30 caliber barrels have had virtual identical bore and groove diameters; that really helps.

 

[Trickymissfit]

Some myths are mentioned in this thread. I'll try to dispell a few.

A barrel whips, vibrates, wiggles at one frequency based on its weight, caliber and profile. Doesn't matter if pipsqueak loads or elephant busters are used, the barrel vibrates at the same frequency for every shot. Most centerfire rifle barrels have a resonant frequency at 30 to 100 Hz (cycles per second). That's the frequency that causes the greatest angular displacement at the muzzle's bore axis. Harmonics of this fundamental are 2, 3, 4, 5, 6, 7 and 8 (and more) times higher in frequency, but they wiggle the barrel much less in amplitude and are, for all practical purposes, negligible.

A weight on the muzzle changes the frequency a barrel vibrates at 'cause the barrel is now longer and heavier. These are popular on smallbore rimfire rifles 'cause they do work. But not needed if your particular lot of .22 long rifle ammo shoots no worse than 3/10ths MOA at 50 yards and 7/10ths at 100; that's as good as today's ammo and barrels produce.

That basic frequency doesn not change as the barrel heats up in most shooting events. The barrel will droop from softening from high temperatures when it does. If it's that hot, the cartridge will fire as soon as its chamberd; probably before the bolt's closed.

Properly heat treated and stress relieved barrels fit to receivers whose face has been squared up with the chamber axis do not change point of impact as they heat up. Too many folks testing handloads in match rifles using machine rests shooting 1 shot every 30 to 40 seconds putting 20 to 40 bullets inside 1/2 MOA at 600 yards kind of prove this. Arsenals testing 30-06 and 7.62 NATO match ammo shoot a couple hundred shots per test group in as many minutes or less. Ever seen a 200-shot group of bullet holes about 6 inches in diameter that was fired at 600 yards? High power long range competitors often put 25 to 30 rounds into a 1000 yard target during a 30 minute time limit for the match. They don't have a zero change; none whatsoever. Such events are shot with heavy, stiff "bull" barrels on some rifles and shorter, flimsy servie rifle barrels in others. They all shoot to the same point of aim as they heat up so darned much you'll burn your pinkie if you touch one; especially the service rifle ones. They get so hot a round left in the chamber for more than 15 seconds will heat the powder up enough to shoot the bullet out faster and it'll strike high.

Why do so many factory rifles walk bullet impact as they heat up? Their receiver face ain't square with the chamber axis and as the barrel heats up, extra pressure at the high point caused the barrel to whip in a different direction. The hotter it gets the more the barrel and receiver steel expand putting more pressure at that point making bullet go even further away from point of aim. And if the barrel's not properly stress relieved, that just add more impact error as it makes the barrel bend.

To summarize, rifle barrels are like steel strings on a musical instrument; guitar, violin, mandolin, banjo or whatever. For each one without a finger pressing it against a fret, plucking it makes one note; it vibrates at one frequeny. Doesn't matter if a fly barely plucks it with its left middle leg or an elephant swats it with its trunk. One will make a barely audible sound and the other very loud. The note's the same but the volume's different because it's whipping at different amounts. Press that string against a fret and its effective length is now shorter and it vibrates at a higher frequency.

going back to the actual frequencey a barrel vibrates at a second. If you happen to have access to a vibration analysis piece of equipment you will most like get a 60Hz reading right off the top (no round being fired and the piece just setting there static). In the UK and other points on the planet you will see a 50Hz figure. This actually comes froms AC electricity going thru the building or even out doors if it's close by. Yet if you were on DC current you would see something 100% different. There have been papers pointing this out many times, and for a long while it liked to drove me nuts eleminating something that you can't. You can take two bars of steel that are of the same composition, and the actual readings in Hz will be different as well. A piece of steel that is longer (say 20%), but the same deminsions will tend to show more of an enhanced vibration, but yet at the same frequencey (like it's been amplified). Yet some alloys are very prone to resist vibration. When a bullet (in this case) is driven down the barrel it will have a shockwave following it, and another proceeding it (they will be different). Whenthe bullet strikes the rifeling it will cause the barrel to react in a twisting motion (normally clockwise) as it travels thru the bore. But behind the bullet the barrel is trying to unwind from the bullet trying to induce a twisting stress. The slower the twist rate the less of this you'll get.

Tuners (they call them "tunas") work in two different concepts (last time I heard anyway). One of them uses a slug to change the weight of the barrel out past the muzzel, and the other works a little differently than the added weight, but in the end makes the barrel seem longer as well. One also works to aid the bullet's escape from the following shockwave. I built several of the latter kind, and they do work well. The one with the weight was the next adventure, but my design never got past the drawing board, and I wasn't sure if they would have been any better anyway. The material used seems to have some effect. Most are made of steel or aluminum, but have found dead soft brass (never bronze) to be better. The adjustment thread is tricky, and I think I settled on either 56 or 60 threads per inch. The really hard problem you encounter is not the tuner itself, but mounting it without messing with the bore's integrity. I think one would be best served by setting the barrel up for the tuner before ever gun drilling the barrel. My design looked a lot like a Time Precision on the outside, but was much different in it's workings (never saw one till after I'd built three or four). Calfee knows how mine worked, as we showed him. Think I might have gave him a set of prints to build one. The second one that actually uses weights out near the end of it is interesting and very simple. Kinda trickey getting the weight right, and the weight will change per bullet manufacturer, and even velocity. I sorta figured that the best way to mount the slug (that's what we called them) was under total compression rather than the normal three or four small screws. The slugs are really nothing but a piece of metal that resembles a generic washer. Their alignment is important, and the bore size should be critical as well. I also found that while running some test in a mechanical CAD program that the three or four screws holding the plate didn't work all that well, and had a tendencey to actually creat a vibration itself. But seven 3mm screws worked pretty well, a compression device to hold it in place was much better yet (plus some more stuff). They also seem to work better with smaller diameter barrels than they do with big heavy barrels (first ones anyway).

In a rebuilt Cooper that had a Win. 52D chamber, and some mods done to the extractor system. Plus many other small mods here and there Tony shot mid to low fours constantly, and would dip into the mid threes on occassion with out a tuner. With the tuner and several different kinds of ammunition the gun shot mid twos very consistently, and did a few groups in the high ones (think .170" was about it). What we really saw was that although the groups shrank the shot to shot performance was much more consistent. Tony was able to make ten shot ragged holes. Even the cheaper stuff shot very well (Greentag, etc), and like I said before Tony did more than one top five finish on the Borden Saturday night shoots. But we also learned that the Cooper just wouldn't run with Calfees XP100s and the Harts and Time Precision actions.

It was a fun experiment over a 24 month time frame
gary

 

[Trickymissfit]

Interesting Bart.

I shoot a .223 in 100 yard comp and it has an adjustable harmonic damper that I actually mark (the barrel) as to what loads do what so I can index the damper according to load and bullet weight. The 'sweet spot' varies with round chambered and it don't change from cold bore to warm.

People ask me what all the paint marks are for all the time.....

there are people that have played around with tuners for use on a centerfire rifle. I have never seen one, so I can't say much about them. I've played around with the .223 for way too many years, and have found that a typical varmit barrel length shows up at around 20" to 21.5" (#5 or a #5.5). I agree with Bart in the fact that the node for the blank will be similar wether you shoot a .221 or a .222mag in it. How much a 6mm bore will change the node in a similar sized blank I don't know, but it will move a little bit. I have not rang a barrel since 2004
gary