Does Barrel Vibration Exist and Does it Matter

[CharlieNC]

OK. An ongoing question. A lot of information and misinformation is out there. Yes it matters. The attached link to a Paper by FN covering a redesign of a SAW barrel is probably one of the best that is available that demonstrates the impact.

https://apps.dtic.mil/sti/tr/pdf/ADA434693.pdf

Good info. The high speed video demonstrates the large cantilever beam frequencies everyone likes to reference play no role in actual accuracy considerations because the bullet has already left the barrel before they kick in. On the other hand there is a faster transient cycle around the bullet exit time which matters. Interesting.

 

[big5ifty]

Barrel = tuning fork.

^ nice and simple.

I would add

barrel block = tuning fork dampener

 

[charlie b]

You can see two different frequencies at work there, especially near when the bullet leaves the muzzle. Frequency domain wise it does not seem to track with either OBT or bullet motion theory. I wonder if the smaller frequency has to do with the bullet passing the gas block and subsequent large changes in combustion pressure.

But, the time seems off. Everything I read about before, the bullet leaves the muzzle in 1-2ms (.001-.002s). The data plot shows almost .010s (10ms) for a bullet exit.

It is interesting that this data supports the previous FEA analysis done by another member of the forum.

And, yes, adding weights or pressure points changes the data.

There is a company out there claiming to have made a 'dead' barrel, it has no harmonics. Bonded barrel jacket of largish dia with a lot of longitudinal holes of specific design. Their claim is it needs no tuning since the barrel has so little movement to it. Kinda like the idea of the carbon fiber jackets but on a more robust scale,

 

[jelenko]

But, the time seems off. Everything I read about before, the bullet leaves the muzzle in 1-2ms (.001-.002s). The data plot shows almost .010s (10ms) for a bullet exit.

I was wondering about that also.

 

[SD guy]

Find articles on Optimum Bullet Time (sometimes referred to as Exit Time) that provide information on the way to tune loads to nodes in the vibrations.

I find that the worst to best conditions produces about a 0.090 inch variation in group sizes at 100 yards, (with the same powder, primer, bullet, and brass).

For me, the best condition occurs when the shock wave is at the chamber when the bullet is exiting the muzzle. The worst occurs when the shock wave is at the muzzle when the bullet exits. It not only has the largest vibration but the distortion of the crown is the greatest. At least, that is the theory.
For me is seems to work.

Two questions come to mind here. How heavy / what diameter & length of barrels have you been working with, regarding the .090 variation in group size?

How do you determine when the shock wave is matching up with the bullet exit time?

 

[Doom]

You really need a program like GRT (Gordon's Reloading Tool) or Quickload to di it well. Both programs have OBT calculations.

Optimal Barrel Time Concept

 

[ebb]

I have a 6.5-284 built by a smith that routinely builds national championship winning rifles, it shoots pretty good. Jon Newman, a member here that passed several years ago, proved vibration to me. We went to the range and he brought his reloading gear. We were trying a new powder N165. He loaded ammo that was .10 of a grain hotter with each case. I watched as I fired each shot at the same POA get farther and farther up the target, then 4 at almost the same height. He continued to load and the impacts started to get lower with each shot. I had done OCW tests before but shot 3 shot groups, and I think this has too many shots and blurs the results. I think the test can produce better results with one shot for each load, but only with a good rifle with a match barrel, the results are much more obvious. I still use the 3 shot test with factory barreled rifles with less than match barrels.

 

[CharlieNC]

Yes a ladder test conducted with small differences in charge weight across a range of 5% or more will exhibit a flat spot or even a down turn in poi, this is very reproducible and not simply noise. In Vaughn's book he presents a procedure to analyze this poi data which can be related back to the muzzle vibration frequency, which is the speed of sound in steel at that barrel length. Yes that is also the frequency utilized in the OBT method, but it is muzzle vibration vs diameter changes. These vibrations are much, much faster than calculated using the traditional cantilever beam approach. This all adds up to yield a consistent picture.

 

[CF Junkie]

Doom is right. Personally, I have used QuickLOAD for at least 14 years.
QuickLOAD provides the exit time at the muzzle for every load variation, if you enter the correct barrel length for the rifle you are loading for. The exit time of the bullet depends upon caliber, bullet weight and shape, barrel length, powder, powder charge, trim length and seating depth. I calculate the desired Optimum Bullet Time in order to tune each load to my desired exit time.

Charlie b is also right.
Optimum Bullet Time depends upon the barrel length and the type of steel - caliber doesn't matter because the reflection goes through the steel, not the hole in the barrel. But caliber, of course, and bullet weight change the parameters of the load, but the Optimum Bullet Time for the barrel is based upon the barrel.:
18-inch 416R SS barrel, the exit time at the 12th reflection is 0.897 msec.
22-inch .3% carbon steel, the exit time at the 12th reflection is 1.151 msec.
24-inch .3% carbon steel, the exit time at the 12th reflection is 1.256 msec.
24-inch 416R SS barrel, the exit time at the 12th reflection is 1.199 msec.
26.125-inch 416R SS barrel, the exit time at the 12th reflection is 1.305 msec.
28-inch 416 R SS barrel, the exit time at the 12th reflection is 1.399 msec.

I compared the 12th reflections because they don't flirt with Pmax.
The 18-in 416R SS barrel is well into Pmax with bullets over 65 grains if I use the 10th reflection at 0.750 msec.

As charlie b indicates, in no case does the bullet exit time exceed 1.4 msec unless you load for the 14th reflection, which for some bullet weights gets close to published minimums. Even then, a 28-inch 416R SS barrel only gets to 1.632 msec ET at the 14th reflection.

 

[charlie b]

There are two sets of motion (at least) going on here. The first is the cantilever and the other is the longitudinal sonic shock wave in the steel of the barrel.

The cantilever is a larger movement overall, as the charts showed. Another member in here did another FEA of the effect with different weight barrels and tuners (barrel end weights). This is basically a 'slow' movement, but, it starts before the bullet leaves the barrel. The motion is caused by the gas pressure and bullet traveling through the barrel. The larger effects of it we see in the form of barrel 'whip', especially in longer, thinner barrels. There are some videos out there of the FN FAL on full auto that makes the barrel look like it is made of rubber. Newer steel and liners, as in the article, help to mitigate that, as well as s bit thicker barrels. Gas blocks in the right places also dampen the movement.

The longitudinal shock wave is less 'robust' but acts on the diameter of the barrel, causing it to change in diameter as the shock wave travels through the bore (this is the originator's theory). The problem with theory and actual practice is that the shock wave is not a 'clean' one. Starts with primer ignition, then powder ignition, then a ramp up in combustion pressure. I find it interesting that it predicts nodes at all given the 'dirty' excitation curves, but, it does work to some extent. I do find I have to fine tune the results, but, I don't know if that is because of the inconsistencies in the internal ballistics calculation or the theory. FWIW, the originator of the theory speculated that the 'node' would only get you within 1 MOA, but, some of us have seen better than that in some instances.

My opinion is there is a third motion due to the longitudinal shock wave that is on top of the cantilever effects. The longitudinal shock is presenting as a stress along the length of the barrel (not as much in diameter as the originator presented). This should be equal on top and bottom but gravity has an influence here. So, in essence you have two frequencies 'bending' the barrel, slow but 'strong' and fast but 'weaker'. Where the peaks coincide you get higher POI. When the lows coincide you get a lower POI.

 

[ogre1911]

Yes. Yes.

 

[butchlambert]

Back in the olden days just after the last dinosaur died. I wrapped an unlimited barrel with lead golf shafting tape. It worked.
You might peruse Varmint Al's research.

Varmint Al's Eclectic Web Pages

 

[JFrank]

If barrel vibration did not exist, would all our tuning ladder bullets just go vertical and not cluster ? I image short range would just have one point of impact.

 

[209jones]

Lots of interesting debate around on the subject of internal/external ballistics and relationships, lots of things we do know, many we think we know, lots we aren't really sure about, maybe never will be, due to so many variables involved. Do wonder at how many billions have been spent in studying it over the last 150 yrs or so, and how many more will be spent on it, and to some extent, how many minds have been burnt out on it, mostly because those variations in various components of what it takes to launch a bullet and the very realistic probability we never really will know, exactly how all of them really work together to produce what final result. Knowledge is a fickle bitch, ain't it?

 

[Doom]

If barrel vibration did not exist, would all our tuning ladder bullets just go vertical and not cluster ? I image short range would just have one point of impact.

No. Groups might be smaller but there are a bunch of other variables that change shot to shot. These contribute to dispersion.

 

[charlie b]

Back in the olden days just after the last dinosaur died. I wrapped an unlimited barrel with lead golf shafting tape. It worked.
You might peruse Varmint Al's research.

Varmint Al's Eclectic Web Pages

Varmint Al's FEA work was what I referred to above.

 

[butchlambert]

Varmint Al's FEA work was what I referred to above.

I guess I missed the part of him getting recognized in your post.

 

[Fire Dog]

View attachment 1696094
The idea of wrapping aluminum around a barrel has intrigued me for sometime, I’ve read about it in rimfire sports and figured why not give it a whirl in a centerfire. It was an all day affair to accomplish the wrap the way I wanted to but the results have been good. I wouldn’t give too much credit the wrap because it’s only been tested very little but has won all the matches I shot it in. Another couple barrels this way with similar results would give better data. 300 wsm in f class.

How is that sheet of 6061 joined? Did that require a lot of hammering with a soft mallet?
What is the thought behind using the wrap? Is it just a barrel length tuner or work on some other sort of principle?
Long barrel or a short PPC special which are {in my observation} about 22” long and stout say 1.25”? A would think a short, stout barrel would have very short wave and high frequency vibration.
Just have to ask…. Is there an accessory hat made from the same material to block out mental interference from other shooters???

 

[butchlambert]

How is that sheet of 6061 joined? Did that require a lot of hammering with a soft mallet?
What is the thought behind using the wrap? Is it just a barrel length tuner or work on some other sort of principle?
Long barrel or a short PPC special which are {in my observation} about 22” long and stout say 1.25”? A would think a short, stout barrel would have very short wave and high frequency vibration.
Just have to ask…. Is there an accessory hat made from the same material to block out mental interference from other shooters???

Was it Jackie Schmidt or Jerry Hensler that epoxied a sleeve on a barrel?

 

[Fire Dog]

Varmint Al's FEA work was what I referred to above.

Varmint Al’s site is where I remembered the short video illustrating the subject matter being discussed in this thread about barrel vibration/harmonics.

Barrel Harmonics Mode Shape Movies