Barrel Harmonics Velocity Cycle?

[Boomer Mikey]

Over the years I've noticed a particular powder charge in several guns delivers excellent results with several different bullets of similar design even when bullet weights vary greatly.

Another atribute of these loads is that velocity is usually within 50 fps or less for all bullets and bullet weights and point of impact on targets is almost always similar.

I'm lazy, I usually charge a group of cases with the same powder charge and load two or three different weight bullets then shoot the bunch at a single range session with each weight bullet on it's own target... all the 165's on one, all the 200's on another.

For example: 21 grains of 2400 in my Savage 308 shoots well with RCBS 30-165-SIL, 30-180-SP and 30-200-SIL bullets. Groups are between .7 and .9 inches for 5 shots and velocity is between 1913 and 1934 fps with bullet weights of 173, 185, and 206 grains.

I've experienced this with jacketed loads as well in too many rifle and handgun loads to list while using moderate charges of powder.

It's interesting to lay test targets on top of each other with the same order of powder charges and see similar group characteristics for each powder charge and different bullet weights.

45 2.1 reminded me about the load development accuracy cycle the other day.

45 2.1's words:
"The cycle starts with a vertically stringing group which will tighten up as charge goes up to a bughole group, then starts stringing horizontally as the charge increases, then the cycle will repeat itself. All through the wonder of barrel harmonics. These seem to repeat at about 300 fps cycles in the shotgun speed powders."

The powder charge appears to develop the barrel harmonics and velocity that produce accuracy more so than bullet weight. Is there a barrel harmonics velocity cycle?

I wonder if you experienced this during your load development too?

Boomer :Fire:

[docone31]

I am not even close to seeing if that is true. I am still working on sizing to fit.
I have read about the theories though.
I like the ones where the barrel rings and heterodynes. It makes sense. Almost like a pressure wave under a solid surface. Out of balance and chaos. As long as the wave is in balance, predictability.
I am going to enjoy this thread.
Barrel harmonics.

[runfiverun]

in my 25-06 i shoot the same amount of 4831 under 85 through 120 gr boolits.
i have done this in a few other rifles also.
i have noticed also that when using the same boolit in more then one case
often the same load will work in them. 308, 7.5x55,7.65 arg, 30-40 etc.
seems sometimes that not exact but close enough in case size is good enough.

[Bass Ackward]

Is there a barrel harmonics velocity cycle? I wonder if you experienced this during your load development too? Boomer :Fire:



I vote yes. But it is easier to discern per specific rifle than trying to layout a general pattern. Cause the variables involved in this makes a dramatic difference.

Proof is factory ammunition which strives to find a common harmonic point across many guns in a particular caliber and bullet weight. But that same ammunition seldom works as well in all guns. Hodgdon lists a youth load for 4895 of 41 grains in a 30-06 with 150 grain bullet. 41 is a point, 46 is, and so is 49 in my gun. 49 is the best, 41 second, and 46 is last. In my wife's 06, 41 is the best, 46 second and 49 the worst. So since 49 is max, she requires another powder to hit the top vibration pattern best.

With lead, it can be harder to establish these points because they are wider and they move. The slower the powder you use to be easy on the lead, the wider those points in terms of velocity, will be. Lead friction is not constant as it changes, especially when working at the cusp of a hardness / lube combination. Temperature and fouling are BIGGEST variables here. Which is why you hear of good groups with three shots, or 5 shots, but seldom of 10 or more. Temp and fouling begin to alter friction enough which alters the vibration pattern effectively walking the rifle away from the load or changing POI.

Instead of working the load up for the rifle, I effectively tweak the rifle to the load. With lead, the secret is keeping the rifle from heating and walking away from the load as you shoot. This is easiest done at low pressure / velocity. If you want a load that performs best with warm temps, you are best served working it up that way.

[BABore]

45 2.1's words:
"The cycle starts with a vertically stringing group which will tighten up as charge goes up to a bughole group, then starts stringing horizontally as the charge increases, then the cycle will repeat itself. All through the wonder of barrel harmonics. These seem to repeat at about 300 fps cycles in the shotgun speed powders."

The powder charge appears to develop the barrel harmonics and velocity that produce accuracy more so than bullet weight. Is there a barrel harmonics velocity cycle?

I wonder if you experienced this during your load development too?

Boomer :Fire:


Yep! All the time. I shoot a custom target I designed on CAD and plot out on 24 x 36" paper. It has 3 rows of 5 targets each with 1" grid lines. Targets are numbered 1-15. I test my incremental powder charged loads by the target number. This makes things very easy to see and compare. I usually take a digital pic of the target for safe keeping, and also record the data on the load notes which get transcribed into a journal. I make a layout of the bullet holes for each group. Do it enough and you will see this same repeating pattern Bob talks about. Incidently, Bob told me about this stringing pattern a couple years ago. I just confirmed it myself.

A similar thing occurs when shooting a ladder test. You will see the individual shots climb steadily then pause for several shots. The next incremental charge may actually drop down when it should have went higher. Sometimes it will shift side to side.

It all starts out with stringing due to burn pressure with bbl harmonics muddying the waters. When proper burn pressure is reached, then its primarily harmonics. Jacketed bullets show higher frequency in swings than cast due to hardness and friction differences.

Bass brute forces the load by adjusting the gun. He must be stubborn and only have one powder.:-D I switch powders until I find one that works. They both get you to the same place.

[felix]

Remember, it is the acceleration profile of the projectile which causes the vibes, and not the velocity at any on point within the barrel. Just changing the primer will make things go astray at the target. Luckily most of our guns cannot repeat the pressure curve well enough to make a primer change obvious. Changes in ignition more readily affects the vibes because the barrel has more time to react. A radical change in pressure towards the end of the barrel will never be noticed at the target, assuming the projectile and muzzle are perfectly uniform. Compare this with the cross-wind effects at the muzzle versus at the target. More deviation can be expected when the wind is strong at the muzzle versus at the target. Keeping this in mind, it becomes rather ridiculous trying to trickle powder with the medium burn and slower burn powders. A half-grain of powder variance will never be detected. ... felix

[Boomer Mikey]

Bruce and felix have answered several of my "next" questions.

I think the ladder test is (unintentionally) one of our best kept secrets; it's a valuable tool that supports what felix is talking about. I've never been able to see on target differences between thrown and weighed charges with the medium or slow powders either. I like the concept of finding the range of charges that come together with a barrel harmonics "sweet spot"... with that knowledge, one should be able to produce loads that are stable over a range of temperatures and the minor powder charge variations typical with metering extruded powders.

The acceleration profile appears to be the technical term for what I call the powder charge-velocity relationship.

Thanks for including primer selection into the big picture felix.

Boomer :Fire:

[405]

I'd vote yes also. BUT, putting quantitative numbers or predictions to it is another matter. I know I don't fully understand it. I think I've seen it in enough high powered, very accurate, scoped guns shooting jbullets that it IS real. Predicting it in those guns seems always elusive. The problem I have with allocating cause-effect to accuracy, POI, group shape, etc. with cast bullet loads is that there are so many other factors that affect where the bullets go. Singling out and saying it is harmonics AND/OR any large number of other things seems a stretch. Too many other "masking" events affect cast. That isn't saying hamonics doesn't affect cast.... assigning cause-effect to cast shooting seems extra difficult.

One interesting thing I read a while back has to do with barrel vibes. I think for a long time most shooters thought of barrel vibes acting on the barrel as a fixed-end cantilever.... where the muzzle whipped about in some pattern during the shot. Additionally, can be described as about a cycle and a half sine wave.... or something. BUT, one fellow also came up with (maybe backed up with some hard evidence) that in addition to the sine wave vibe there was also a very high velocity compression wave that travelled back and forth along the axis of the bore during the shot... forming something like a donut ring as it travelled. If the bullet exits the muzzle at the "wrong" time in conjunction with this "donut" it would skew bullet departure angles and hurt accuracy. If the bullet exits the muzzle at the "right" time it would not hurt accuracy. So instead of the one type of vibe affecting accuracy there may be two types going on at the same time. SHEEZ! no wonder one hole groups are only a goal punctuated by a series of rarely obtained objectives :mrgreen:

[felix]

Yes, indeed, there are two energy waves transversing the barrel. The very first wave is set up by the hammer falling onto the primer. This vibe can become additive at any time during the vibe set up independently by the ignition, making things immensely complicated on where a projectile sees an aberration from one or more combo vibe peaks. Hopefully, the vibe set up by the hammer is perfectly consistent throughout the group formation on paper. Won't hurt to oil the firing pin mechanism and trigger with Jojoba every once in a while, after flushing with lighter fluid. ... felix

Any kind of compression wave set up by the projectile progressing towards the muzzle would have to be consistent enough to disregard in terms of harmonics. The trigger and firing pin, maybe not. ... felix

[crabo]

What exactly is the "Ladder Test" and how is it done?

[BABore]

What exactly is the "Ladder Test" and how is it done?

You work up a load in 0.25 (best) or 0.50 grain increments. One round at each charge weight. You shoot them at 300 yard (best) on one big target. I use 24 x 36 plotter paper with 1" grid lines and a single large diamond near the top. Fire a round, mark the shot #, and leave the gun cool down to ambient temp., then do it again. You will see the rounds impact slightly higher with each additional increase in powder. At some point you will see sucessive shots cluster, then move on again. Sometimes, after clustering, the next shot may actually drop lower, then they start climbing again. Depending on increments and min/max powder spread, you may see the clustering several times. Changes in windage are somewhat ignored for the time being, but they can be indicators as well. You note where shots clustered and load your 3-5 shots for these charges. That's where your most likely to see the combination's accuracy. May or may not be as accurate as you want, but it saves alot of 3-5 shot groups at each increment.

It's not for everyone cause it takes time, patients and good technique. You can't call flyers here. You only got one shot. The longer range is to give you a better spread or expand the scale. It's hard to tell what's going on at 100 yards. 200 is not too bad. My shooting partner and I use walkie-talkies to stay in touch so he can mark shots. Only has to walk 50 yards instead of 300.

[Whitespider]

At the risk of being (virtual) pistol whipped, I’ve never bought into the ladder test/barrel harmonics thing. I load to MY desired velocity level and “tweak” until uniformity and accuracy is acceptable to ME. I use a chronograph to measure load uniformity, and 9 times out of 10, if the uniformity is there so is the accuracy. But, I do start with a bullet or boolit that my barrel likes, and that seems to be most of the battle. I don’t waste time lookin’ for any “sweet spot”, not interested. I know the specific performance level I need or want before I start, anything less is unacceptable! The “ladder or incremental test” is used only to check for pressure problems on my way to the desired performance end. Could be as I’m “tweaking” the load I’m also “tweaking” the harmonics, but I ain’t gonna vary from my intended performance goal because of some perceived “sweet spot/harmonics/ladder test/whatever crap.

[runfiverun]

the way i have always thought of a sinewave was as a donut,
being pushed down a bbl by a bullet,
it is funny that i thought of this when shooting a heavy bbl'd 223.
as the thickness of the bbl was about 10 times more then the bullet.

[405]

the way i have always thought of a sinewave was as a donut,
being pushed down a bbl by a bullet,
it is funny that i thought of this when shooting a heavy bbl'd 223.
as the thickness of the bbl was about 10 times more then the bullet.

It's so far beyond what most of us deal with it becomes mostly academic pretty quickly.

I guess the way I've always thought of basic barrel vibration is something like: Take a steel rod and attach one end to something solid (basic cantilever). Tap the rod or the solid base. Viewed from the side... the rod at any one instant will take on the shape of a standing wave- a shape similar to a sine wave ~. Depending on when the bullet exits.... the muzzle will be pointed in any number of directions. After analyzing barrel vibrations with better equipment researchers have found that the barrel vibrates not only on one plane but also usually in a circular pattern. Of course the barrel (cantilever) vibes are modified by how it's attached to the stock- enter stock/action bedding techniques- forend pressure, uneven pressure, changing pressure, free floating, etc.

The "donut" analogy of a high velocity pressure wave (P wave) was more recently confirmed with yet more equipment. That wave travels up and down the barrel at extremely high velocity. Yet more testing has shown many secondary vibration sources and that all these vibrations and "waves", due to firing and bullet travel, are very chaotic and complex.

The "ladder method" for isolating best loads may be a way to find the "sweet spot" for accuracy in all this chaos. Pretty much beyond my complete understanding though. :confused:

[BeeMan]

This is an interesting site with engineering analysis of barrel stiffness, vibration, etc.

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

[felix]

Yes, I debugged a computer program for finite analysis doing the design of a transmission-line tower for Houston Light and Power. Indeed, it is fun applying such a program when building an object with variable load dynamics applied to the object. However, this approach is used primarily to construct an object having many sub-objects, like doing different independent sections for a tower.

The analysis of the barrel should be done as if the barrel was one composite object. The most expedient approach would be to interject transient loads on the barrel at different locations
and read the vibe pattern. Take the Fourier transform of that wave form, creating the one or more vibe patterns which are subservient to the whole. For each of those, obtain the S-plane characteristics (Laplace transform) and define an equivalent electrical circuit which would produce that pattern. Then, using the circuits, a complete simulation could easily be accomplished of the entire barrel reaction by: hitting the primer, the ignition, the projectile movement, and so-on. Anything short of this mode of operandi is nothing but entertainment and about worthless in determining how to fix the entire vibe pattern given a set of circumstances.

Been there, done that.

... felix

[BABore]

Yes, I debugged a computer program for finite analysis doing the design of a transmission-line tower for Houston Light and Power. Indeed, it is fun applying such a program when building an object with variable load dynamics applied to the object. However, this approach is used primarily to construct an object having many sub-objects, like doing different independent sections for a tower.

The analysis of the barrel should be done as if the barrel was one composite object. The most expedient approach would be to interject transient loads on the barrel at different locations
and read the vibe pattern. Take the Fourier transform of that wave form, creating the one or more vibe patterns which are subservient to the whole. For each of those, obtain the S-plane characteristics (Laplace transform) and define an equivalent electrical circuit which would produce that pattern. Then, using the circuits, a complete simulation could easily be accomplished of the entire barrel reaction by: hitting the primer, the ignition, the projectile movement, and so-on. Anything short of this mode of operandi is nothing but entertainment and about worthless in determining how to fix the entire vibe pattern given a set of circumstances.

Been there, done that.

... felix


Whoa! That made my head hurt.:???:

Being a welding engineer, I'd just weld the barrel to a section of 12 inch H-beam and call it good to go.:-D

[felix]

Yeah, me too, BAB! That is why I retired several years ago. Of course, these days it does not take much for me to retire to something else, and, as a result, there are many unfinished projects around this house. Like cleaning up my reloading room. It has not been cleaned in 5 years. ... felix

[Harry O]

I have used the "ladder test" described here several times through the years. I called it the "T-target", myself. I got some butcher paper and draw a line down the middle of it. I tape a regular target on the line at the top and measure & draw cross marks every three inches below the center of the target.

I load up (and mark) a bunch of cartridges 1/2gr apart starting from maximum going downward. I shoot them (usually at 200 yards), look at them with a spotting scope and record the EXACT position in a notepad. After I am done shooting, I retrieve the target and mark the loads directly on it (reading from the notepad). Then I look for groups of three or four adjacent-load bullets. When I find one, I load up some more cartridges midway between the top and the bottom load on the T-target group.

It is VERY rare (from my experience) that the maximum load is also the most accurate load. Playing with the seating depth, crimp, or other things doesn't do nearly as much for accuracy as finding a sweet spot.

I read about this test many, many years ago. It was named after the guy that popularized it, but I have forgotten the name. I am convinced that it is the quickest way to find an accurate load in a new rifle.

[runfiverun]

the only bad thing about the ladder method is that, in at least one of my
rifles, is that the faster loads actually tend to shoot lower.

[Bass Ackward]

the only bad thing about the ladder method is that, in at least one of my rifles, is that the faster loads actually tend to shoot lower.



POI can change any direction based upon the oscillation pattern of the barrel which is why I only follow group. I have seen barrels move up and down. But I have seen them move up right and down left too or visa verse as the pattern becomes elliptical. Some of this can be scope as you get farther from the aim point. This is why I love the bubble. I suppose this is also why a poor performing barrel can be re-chambered and become an excellent one in the new caliber.

Barrels rarely rotate consistently as change with powder / bullet combinations and fouling. And temperature will have an effect too. This is where a smooth interior finish and a loser bore helps as friction is increased with velocity and pressure, but at a much lower rate. So then this pattern is narrowed what ever it is.