OBT theory question

[rammac]

"Match grade barrel makers like a 12 to 16 microinch surface finish. Any smoother and the surface starts wiping off bullet jacket copper unevenly around them. Friction increases. Accuracy degrades."

Hmm, it would be interesting if you would show proof of this rather than refer to a question and answer web site that was founded by a Facebook employee. Even so, if you read further down the page of your proof source it refutes your claim and states that the friction increases due to the surfaces becoming so smooth that there isn't any room for air and a vacuum is produced, which is exactly what I was taught as a machinist and is the theory behind vacuum tables.

But the overarching truth is that in the real world you'll never be able to find a bullet with a jacket that is smooth enough to cause this problem. The barrel can be made far smoother than the bullet will ever be (even standard mass produced barrels are smoother than bullet jackets) so the rougher surface of the bullet will always slide over the less rough barrel steel.

 

[dcali]

I've heard barrel makers say that there is an optimal finish, and that it's possible to make them too smooth. I see no reason to dispute them. And I believe the 1.5 inches that Bart is talking about is that cartridge length is not accounted for. A shorter cartridge will have a longer barrel time than a longer one given the same barrel length. That's probably the least of OBT's problems, though.

 

[Bart B.]

And I believe the 1.5 inches that Bart is talking about is that cartridge length is not accounted for. A shorter cartridge will have a longer barrel time than a longer one given the same barrel length. That's probably the least of OBT's problems, though.

Well now, simple reasoning has its merits.

To say nothing about the probability your load using different component lots and barrels Quickload did to calculate their stuff will match yours. No wonder arsenals use different charge weights of the same powder type to meet velocity and pressure specs, sometimes near a 2 grain spread with the same test barrel specs.

 

[CharlieNC]

I belonged to professional organizations that utilized member funds to research key topics of common interest. Wouldn't it be informative to pool Accurate Shooter members to contribute into a fund to measure exactly what is going on with barrel harmonics that define a node as we continue to identify via ladder testing. Initially this would focus on mapping out what really happens, not the theories , but the real world results to dispell all of the debates that continue to rage with no suitable resolution. I bet @dmoran has most of the equipment to do it already!

 

[Bart B.]

I belonged to professional organizations that utilized member funds to research key topics of common interest. Wouldn't it be informative to pool Accurate Shooter members to contribute into a fund to measure exactly what is going on with barrel harmonics that define a node as we continue to identify via ladder testing. Initially this would focus on mapping out what really happens, not the theories , but the real world results to dispell all of the debates that continue to rage with no suitable resolution. I bet @dmoran has most of the equipment to do it already!

Accelerometers and timing instruments would be needed. And the system to receive and analyze data to meet agreed upon objectives, conditions and standards that must happen first in order to determine what stuff is needed.

Varmint Al's web sites may be of help as well as Geoffrey Kolbe's.

 

[Ned Ludd]

Make sure whomever does the tests has a tool to actually measure (not just calculate) longitudinal shockwaves and the resulting cyclic expansion/contraction of the bore at the muzzle. While they're at it, it might also be a good idea to ask them to determine exactly what is happening when seating depth changes.

 

[Bart B.]

Make sure whomever does the tests has a tool to actually measure (not just calculate) longitudinal shockwaves and the resulting cyclic expansion/contraction of the bore at the muzzle. While they're at it, it might also be a good idea to ask them to determine exactly what is happening when seating depth changes.

Maybe this guy is a benchrest competitor.....

http://vibration-consultant.com/?gc...GRM8IUthZxUFGuwwPjMTBuO5d2keQ8QhoCywkQAvD_BwE

Or this one....

https://www.researchgate.net/public...Barrel_Type_Structure_using_Mechanical_Filter

 

[Ned Ludd]

F-Class, not BR, although the way I approach reloading is probably more similar to BR than the typical F-Class shooter. I was merely making light of the fact that there are some pretty fundamental things we routinely do as part of the reloading process that probably haven't been fully explained, or at least, things that you commonly see explained by a variety of different mechanisms in shooting forums as if each had been proven beyond question. I think Charlie's idea of contracting an outfit to study some of these things is a good one and I'd certainly be on board with that. It also wouldn't surprise me if some of these things had already been studied ad nauseum by the military. Unfortunately, obtaining and looking at their results often requires much better Google-fu than I seem to possess.

 

[normmatzen]

First,
Dr. Kolby is Very bright but he specializes in Rim Fire rifles and seems to have a disdain for center fire. And, the tuner function of a rim fire is a bit different than center fire , probably due to the difference in Mv.

Donovan, the OBT theory does indeed depend on resonance! If you consider the barrel an organ pipe, the barrel will resonate as an organ pipe at one frequency. When the shell fires, the explosion initiates a resonance in the barrel longitudinally as a function of the barrel length and the speed of sound in the actual metal used in the barrel. This causes what is sometimes called the moving donut as the resonance causes a slight tightening of the bore that propagates down the barrel, and back, at a cyclical rate. If the cartridge is tuned to this barrel, the "moving donut" causes slower bullets to speed up and faster bullets to slow down which makes the Mv more consistent over a small range of loads. This is covered both in the OBT theory as well as Varmint Al"s treatise.

THEN, you can proceed with tuning your tuner for best cantilever resonance and coincidence!

Norm

 

[Bart B.]

the OBT theory does indeed depend on resonance!

Not the resonant (lowest) frequency but a higher harmonic thereof. Mode 4 in Al Harrel's site is about 4 times higher than resonant frequency at mode 1.

 

[Bart B.]

And, the tuner function of a rim fire is a bit different than center fire , probably due to the difference in Mv.

I disagree. Both tune the muzzle axis frequency in the vertical plane so bullets leave at the correct place on bore axis upswing to compensate for velocity spread's slowest through fastest bullet's drop at target range.

 

[dcali]

Guys, this has been studied and tested by multiple people over the years. We don't need new theory - this is as basic as vibrations get. It's theoretically sound, tested by industries from automotive to aerospace to computer manufacturing, and it works.

Kolbe and Vaughn come to mind first, as they've documented their experiments pretty well. And you can find literature going back 100 years on barrel vibration. All you need are some accelerometers and the will to do the testing. Drop a few grand on equipment, muck about making it all work, and you're there. It's a pain in the rear to get this stuff actually working, but it can and has been done. It finds exactly what you'd expect it to find - that the barrel's dynamic response is like that of any other mechanical object - a mixture of resonant vibration modes excited by the frequency content of the shot. The predominant mode is a (relatively) low frequency side to side whipping of the barrel, which is what we take advantage of when we tune loads.

In other words, the barrel "whips", and we try time the bullet's exit advantageously.

 

[Bart B.]

Kolbe and Vaughn come to mind first, as they've documented their experiments pretty well. And you can find literature going back 100 years on barrel vibration. All you need are some accelerometers and the will to do the testing.

A lot of what happens is easily calculated with the laws of physics. It was first documented over a century ago:

http://rspl.royalsocietypublishing.org/content/68/442-450/327.full.pdf+html

 

[Bart B.]

the OBT theory does indeed depend on resonance!

I think it's based solely on barrel length and the speed of sound timing the shock wave going back and forth in the barrel and when the bullet exits.

A small diameter and large diameter organ pipe of equal lengths produce different sound frequencies. OBT theory does not address barrel diameters, just lengths.

 

[dcali]

I think it's based solely on barrel length and the speed of sound timing the shock wave going back and forth in the barrel and when the bullet exits.

A small diameter and large diameter organ pipe of equal lengths produce different sound frequencies. OBT theory does not address barrel diameters, just lengths.

If I'm reading it right, he's basically talking about a single higher order vibration mode, but it's unclear. Either way, it's not relevant. His dismissal of bending modes is unwarranted - they've been measured, and in fact are dominant. Once he throws that out, the rest doesn't really matter.

 

[dsculley]

OBT and QuickLoad can get you close if you get everything set up correctly in QuickLoad. Since this entails adjusting burn rate and/or other data in QuickLoad to make QuickLoad approximate your empirical data from test loads, then you must have already tested some loads before you can determine where the nodes are based on OBT. Personally, when I have tried to do this I have already identified my node(s) from the test rounds I have fired. I would tweak QuickLoad based on the data from my testing just to see if the identified nodes coincided with OBT theory. Sometimes they did, sometimes not. I tend to be more pragmatic than some of the shooters here. I don't need to know how my truck engine was built in order to drive it. I don't need to know all the theory behind internal and external ballistics to load good ammo and shoot well. Use established and well documented procedures for locating a good node. Use good technique for hand loading. Develop good drop and wind chart. Go shoot!

While i do sometimes read and study ballistics theories that interest me, I find range time more beneficial to my performance.