OBT theory question

[thomseninc]

For those who believe in Optimum Barrel Time theory, I have a question: I just finished a load development on my 6.5CM. It turns out the most accurate load I found corresponds exactly to OBT theory IF I add 1.8 inches to the barrel length to account for my muzzle brake. Is this pure coincidence, or is this something real?

 

[CharlieNC]

Chris Long says brake does not factor into length used for obt. While I have found the method works well to define a node, the proposed longitudinal shock wave mechanism doesn't add up for me. I still have much to learn in this regard.

 

[Keith Glasscock]

You are just in one of the in-between nodes. OBT doesn't account for all of them...

 

[Bart B.]

Chris Long's OBT theory has at least 3 flaws. One is the distance a bullet travels to the muzzle. There's near a 1.5 inch spread bullets travel to the muzzle of a given barrel length that's not considered. Speed of sound in barrel steel and barrel resonant frequency are others.

I've clamped barrel bands on muzzles tight enough to change bore diameters over .000100" and had no change in accuracy. The OBT theory claims .000003" is enough to change accuracy.

Barrel steel expands. 000072" with a 10 degree F temperature change. Not mentioned in the OBT theory article. Accuracy doesn't change as good barrels properly installed heat up.

 

[dmoran]

Chris Long's OBT theory has at least 3 flaws. One is the distance a bullet travels to the muzzle. There's near a 1.5 inch spread bullets travel to the muzzle of a given barrel length that's not considered. Speed of sound in barrel steel and barrel resonant frequency are others.

I've clamped barrel bands on muzzles tight enough to change bore diameters over .000100" and had no change in accuracy. The OBT theory claims .000003" is enough to change accuracy.

Barrel steel expands. 000072" with a 10 degree F temperature change. Not mentioned in the OBT theory article. Accuracy doesn't change as good barrels properly installed heat up.

Your input is incorrect and partially off base.
The speed of sound of barrel steel is in the formulation (pulse wave) of Chris Long's OBT.
Barrel length it uses: from the bolt face to the crown.
Resonant frequency has nothing to do with OBT and does not have any attribute to barrel time nor exit times.
And Chris is referring to bore expansion of the pulse wave, not at all of thermo expansion like your alluding to and trying to read into it.

Have you ever used a pressure trace system with OBT parameters in the OS?
The relativity of the OBT parameters in all the RSI pressure traces that I have ran over the years (since 2006) have yielded significance to there placement.

 

[Mulligan]

Your input is incorrect and partially off base.
The speed of sound of barrel steel is in the formulation (pulse wave) of Chris Long's OBT.
Barrel length it uses: from the bolt face to the crown.
And resonant frequency has nothing to do with OBT and does not have any attribute to barrel time or exit time.

Have you ever used a pressure trace system with OBT parameters in the OS?
The relativity of the OBT parameters in all the RSI pressure traces that I have ran over the years (since 2006) have yielded significance of there placement.

I’m going to read this again in the morning. My head hurts right now.
CW

 

[Ned Ludd]

I'm of the opinion that the explanation may not be correct, but accuracy nodes DO exist very close to where the OBT table predicts they should for a given length. My intuition tells me it's more likely to be a barrel harmonic phenomena, as the OBTs in the table fall within a range likely to fit with vibration frequencies observed in rifle barrels. However, I have neither the expertise, equipment, or motivation to try to prove that.

What I can say is that I tuned several different loads, in different calibers, with different powders, different barrel lengths, and with different weight bullets, before I ever used Quickload, or was aware of OBT Theory. Later, I found that each of these loads was tuned in very close to where they were predicted to tune in based on barrel time. You can believe that's a coincidence if you wish, I choose not to. Having made that choice, it allows me to use Quickload to estimate barrel times and expedite the load development process. That's all. I still rigorously test charge weight/seating depth and let the targets tell me where the optimal tune is located. The use of Quickload and OBT just makes the process slightly faster. So it really isn't a deal breaker for me if the underlying explanation may not be correct. I would certainly prefer to know how it works, but can use it to my advantage regardless of whether the longitudinal shockwave explanation is correct.

To the OP: if the underlying mechanism of OBT does have something to do with barrel harmonics rather than longitudinal shock waves, the addition of a tuner or similar device would be predicted to affect the system similarly to an increase in barrel length. The graph on the following page (about 3/4 of the way down) illustrates several predicted OBTs (red diamonds, just above the X-axis) and their relationship to the point the bullet is predicted to exit the muzzle (colored crosses, on each pressure trace), suggesting that predicted OBTs are at least in the approximate time zone to have some relationship with barrel harmonics (http://www.varmintal.com/atune.htm). Although a tuner or muzzle brake wouldn't affect pressure/velocity in the same way as the tighter rifle bore, it would affect barrel harmonics proportionally to its mass. So I could imagine it might be possible for a muzzle brake to affect OBT similarly to adding some barrel length of mass equal to the mass of the brake itself, although it wouldn't be predicted to be exactly the same because of the differential pressure effect.

 

[dmoran]

.... if the underlying mechanism of OBT does have something to do with barrel harmonics rather than longitudinal shock waves...

OBT is exclusively formulated of the pulse waves and there interaction of bullet travel/timing in the barrel. Harmonics and/or resonant frequency have no relevance or attribute to OBT, that are not equated or formulated in any way to it.

 

[Ned Ludd]

Optimized barrel time (OBT) is exactly that, an optimal barrel occupancy time associated with some accuracy node. It is not the same as "OBT Theory", which is Chris Long's explanation of why accuracy nodes are associated with specific barrel occupancy times. I know what OBT Theory says. Many, myself among them, question whether the explanation that longitudinal shock waves are the reason specific barrel times are associated with accuracy nodes is correct, or at least fully explains the phenomena. I never stated that OBT Theory has anything to do with barrel harmonics, I stated my own opinion that optimal barrel times might have some relation to barrel harmonics. I'm not really interested in debating the OBT Theory topic with you as it's already been beaten to death. However, stating that barrel harmonics have nothing to do with optimal barrel times and accuracy nodes implies that OBT Theory itself is valid beyond question. It is not.

 

[dmoran]

I’m going to read this again in the morning. My head hurts right now.
CW

Clay -
For some aid to understanding my input to OBT, attaching a pressure trace image of mine below, that I edited to include 3 smaller sections of the one complete trace/shot.
- The first section is bullet engagement, with its pressure and timing. This is important to me, that directly reflects bullet release and all the tuneable aspects that effect bullet release and the consistency of them.
- The second section is peak pressure. It's amount, timing, and shape are all tuneable aspects as well.
- The third is bullet exit time, with "OBT Markers" rendered along with the trace timing. Again this is a tunable aspect, and repeatedly have found accuracy nodes and best accuracy, will line up directly or almost directly inline above the OBT Markers, and is why I am a firm believer in Chris Long's OBT Theory and his OBT software parameters significance.

 

[Mulligan]

Clay -
For some aid to understanding my input to OBT, attaching a pressure trace image of mine below, that I edited to include 3 smaller sections of the one complete trace/shot.
- The first section is bullet engagement, with its pressure and timing. This is important to me, that directly reflects bullet release and all the tuneable aspects that effect bullet release and the consistency of them.
- The second section is peak pressure. It's amount, timing, and shape are all tuneable aspects as well.
- The third is bullet exit time, with "OBT Markers" rendered along with the trace timing. Again this is a tunable aspect, and repeatedly have found accuracy nodes and best accuracy, will line up directly or almost directly inline above the OBT Markers, and is why I am a firm believer in Chris Long's OBT Theory and his OBT software parameters significance.

View attachment 1053814

Makes complete sense!
Donovan, thank you for the additional information, it helps a great deal with my understanding of the topic.
Now, I have a few other questions. Ya just gotta love this stuff!!!
CW

 

[CharlieNC]

Clay -
For some aid to understanding my input to OBT, attaching a pressure trace image of mine below, that I edited to include 3 smaller sections of the one complete trace/shot.
- The first section is bullet engagement, with its pressure and timing. This is important to me, that directly reflects bullet release and all the tuneable aspects that effect bullet release and the consistency of them.
- The second section is peak pressure. It's amount, timing, and shape are all tuneable aspects as well.
- The third is bullet exit time, with "OBT Markers" rendered along with the trace timing. Again this is a tunable aspect, and repeatedly have found accuracy nodes and best accuracy, will line up directly or almost directly inline above the OBT Markers, and is why I am a firm believer in Chris Long's OBT Theory and his OBT software parameters significance.

View attachment 1053814

Great info Donovan, got me thinking (scary I know). As we see a cyclic poi behavior resulting from a ladder it is natural to connotate to cyclic barrel behavior, e.g. positive compesation providing a node. Since the pressure is "flat" at the obt as the bullet exits, and velocity results from pressure, could this somewhat explain the poi and node behavior?

You have some cool stuff!

 

[Bart B.]

I never stated that OBT Theory has anything to do with barrel harmonics, I stated my own opinion that optimal barrel times might have some relation to barrel harmonics.

Good reasoning. When slower bullets leave the muzzle at an angle that compensates for their drop at target range because some are faster than others vertical stringing is reduced. Barrel tuners change the several frequencies barrels vibrate at.

Geoffrey Kolbe has a calculator on line that calculates about where your 308 Win bullets leave for different barrel times of bullets.

 

[CharlieNC]

Good reasoning. When slower bullets leave the muzzle at an angle that compensates for their drop at target range because some are faster than others vertical stringing is reduced. Barrel tuners change the several frequencies barrels vibrate at.

Geoffrey Kolbe has a calculator on line that calculates about where your 308 Win bullets leave for different barrel times of bullets.

I have used the Kolbe calculator as a basis for comparison to the cyclic poi behavior from a ladder test, and it did not come close to matching the observed results. On the other hand the obt markers fit quite well.

In reality it appears the Kolbe harmonics describe the gross, macro movement while the higher OBT frequencies provide the nodes we tune with.

 

[Bart B.]

Was the rifle used in OBT tests held the same way as Kolbe's software is based on? Otherwise actual tests will differ

From the software:

"The model assumes that the rifle is essentially in free space and unconstrained by any rests or clamps as it recoils. Shooting a rifle off bags is a fair approximation to this. However, if small calibre rifle is gripped tightly or pulled hard into the shoulder then the recoil dynamics could be affected."

Kolbe's software explains how things happen. It's not intended to show exactly.[/QUOTE]The what your stuff does as you shoot it. Only an approximation.

Two rifles with identical shape and weight except for cartridge chamber size will have different resonant and harmonic barrel frequencies. Kolbe's software uses the 308 Win. A 22 PPC chambered barrel will be stiffer than a 308 Win barrel with the same profile and have different vibrating frequencies.

 

[dcali]

Here's the trouble with believing in OBT as described by Chris Long. It's not correct. There is no amount of opinion involved here. It's just not right, and cannot be right for a whole bunch of reasons.

Now, if it works for people (and many I trust say it does for then - that has not been the case for me), it means that it must line up with vibration nodes, which we know exist. We do know that barrel time is a very important piece of the puzzle. But why Long's theory would overlap with vibration noes, I'm not sure. My suspicion is that either it is just a happy coincidence with a lot of barrels or that people have a wide tolerance for "it works". But if you want an explanation on how a theory that isn't factually sound is impacted by a muzzle brake, then it's kind of like asking if Santa Claus would deliver presents on December 26 if he drank too much on Christmas eve.

 

[dcali]

Kolbe's calculator uses a method I have used extensively as an engineer. It is not going to give you anything approximating real life results. It's a tool to figure out what is happening and how geometry impacts it, not an exact number.

 

[dcali]

Chris Long's OBT theory has at least 3 flaws. One is the distance a bullet travels to the muzzle. There's near a 1.5 inch spread bullets travel to the muzzle of a given barrel length that's not considered. Speed of sound in barrel steel and barrel resonant frequency are others.

I've clamped barrel bands on muzzles tight enough to change bore diameters over .000100" and had no change in accuracy. The OBT theory claims .000003" is enough to change accuracy.

Barrel steel expands. 000072" with a 10 degree F temperature change. Not mentioned in the OBT theory article. Accuracy doesn't change as good barrels properly installed heat up.

I'll add another few.

Longitudinal waves do not change the direction or lateral velocity of the bullet on launch.

If it did work, then you could cut a taper that included a large mass at the muzzle and this would not impact the node. Square that up with how a tuner works.

The surface finish of the bore is greater in size than the calculated bore expansion.

 

[Bart B.]

The surface finish of the bore is greater in size than the calculated bore expansion.

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.

https://www.quora.com/Polishing-sur...of-reducing-it-What-is-the-reason-behind-this

Match grade barrels are lapped to a bore - groove diameter tolerance of about .000100 inch.

All the Garand barrels I wore out had their bore at the muzzle belled out about .002" from cleaning rod wear at about 2000 rounds, no signs of accuracy loss. No copper wash the last half inch on the barrel. None the last 3/4 inch at 3000 to 3500 rounds when replaced when accuracy loss was visible.

 

[rammac]

Interesting conversation, I wished it were understandable...

"Chris Long's OBT theory has at least 3 flaws. One is the distance a bullet travels to the muzzle. There's near a 1.5 inch spread bullets travel to the muzzle of a given barrel length that's not considered. Speed of sound in barrel steel and barrel resonant frequency are others."

What does that mean? Are you saying that the theory doesn't take in to account 1.5 inches of barrel length? Or are you saying that bullet travel can vary by 1.5 inches?

Also, since the OBT theory is based on the traveling waves along a barrel's length, how is the speed of sound in barrel steel and barrel resonant frequencies not considered?