Optimal Barrel Time- Question

[cognac jack]

Do the optimal barrel times match the barrel being at the top and bottom of their vibration cycle? When you run the calculations and get the times is one time the top and the next time the bottom, or are the times just when the harmonics are the best? Thanks

 

[CharlieNC]

Google Chris Long OBT to read the development of theory. But it does not address vertical harmonics, ie up/down. OBT presumes a traverse pressure wave, moving back/forth along the length. This discrepancy is the reason many don't believe it is fundamentally correct from an engineering perspective, although practitioners find it works well.

 

[dcali]

There's Optimal Barrel Time - a theory put forth by Chris Long, and optimal barrel time - just plain English. The former has nothing to do with up and down movement (and is totally wrong, if you ask me). The latter is very real, but it's a lot more complicated than a simple up/down sine wave. That's part of it, but there are different frequencies and dynamics involved that make the muzzle motion not so easy to predict in detail.

 

[Dusty Stevens]

Maybe @gunsandgunsmithing has some answers

 

[gunsandgunsmithing]

Maybe @gunsandgunsmithing has some answers

Damon said it very well!

 

[Ned Ludd]

In order to truly answer your question, you would have to know the exact frequencies of your barrel, as well as understand in detail how the different modes of vibration affect muzzle displacement. I am one of the practitioners referenced by CharlieNC that believe that optimal barrel times are real, but that they are probably not explained by a longitudinal shockwave traversing the barrel. Personally, I believe that the concept of optimal barrel time "nodes" probably fits somehow with at least some of the higher order barrel harmonics, although that has not been proven scientifically. Nonetheless, I have had very good results loading to specific barrel time node over the years. In fact, the results with multiple different rifles and calibers have been far too good to believe it is merely chance.

For a given length barrel, if you know the frequency at which it vibrates, it would be tempting to think you could simply subtract one OBT from an adjacent node in the Chris Long's OBT Node Table and determine whether the resulting difference was anywhere close to (1/2)(1/frequency), or the time it would take to move from the top to the bottom of its arc of travel. Unfortunately, measuring barrel frequency takes specialized equipment not owned by most shooters and the system is far more complex than that.

For example, the frequency of 1st order (Mode 1) harmonic motion at the muzzle analogous to the end of a model cantilever beam is relatively slow, perhaps on the order of 50 to 75 Hz, depending on the barrel and load velocity. That means the time it would take the muzzle to go from the top of its arc to the bottom would be around 7 to 10 ms (i.e. half of (1/frequency). Optimal barrel times would therefore need to be much longer than they actually are reported in the Table, more on the order of 7 to 10 ms, for Mode 1 to be the target. So the explanation of bullets exiting at the top of the barrel arc for one OBT node versus the bottom of the arc for the next OBT node is too simplistic and does not fit with actual measured data. In fact, the frequency of Mode 3 (around 1000 Hz) represents a more likely way in which optimized barrel times could mesh with barrel harmonics.

Another thing to remember is that OBT Nodes as listed in Chris Long's Table are not the only "accuracy" nodes. In fact, they are spaced pretty far apart based strictly on the values in the Table. In other words, velocity from a given barrel length must be increased/decreased substantially in order to hit the next adjacent OBT Node in the Table. If you use a ladder or some other form of load development testing that clearly reveal accuracy nodes, you will find that there are clearly other nodes in-between the values of the OBT Table. In terms of barrel occupancy time, these other nodes are clearly not as far from a given OBT Node as the OBT Node itself is from the next OBT Node.

So what does all that actually mean? To be honest, I have spent some time in the past going partway down that rabbit hole, but ultimately refused to go any further. There are many resources available to you if you wish to pursue some unifying theory between barrel harmonics and the concept of OBT. Personally, I find it much easier to load up a test series, go out and shoot them, and then let the target tell me where the rifle wants to tune in. I'm basically describing the difference between empirical and theoretical information. There are certain things we can readily do to obtain useful information, without necessarily understanding the mechanism of how they work in exquisite detail. So you can easily develop a load that shoots extremely well without using any reference to barrel occupancy time, then go back later using QuickLoad and determine for yourself how well the barrel occupancy time of that load matches some value in the OBT Table. If you do that exercise a few times, it ought to be enough to convince one way or the other whether OBT Theory works in your hands. Personally, I very much like to understand WHY things work the way they do, especially with regard to ballistics, but I often keep coming back to a very simple idea that usually gets me where I need to be:

 

[gunsandgunsmithing]

In order to truly answer your question, you would have to know the exact frequencies of your barrel, as well as understand in detail how the different modes of vibration affect muzzle displacement. I am one of the practitioners referenced by CharlieNC that believe that optimal barrel times are real, but that they are probably not explained by a longitudinal shockwave traversing the barrel. Personally, I believe that the concept of optimal barrel time "nodes" probably fits somehow with at least some of the higher order barrel harmonics, although that has not been proven scientifically. Nonetheless, I have had very good results loading to specific barrel time node over the years. In fact, the results with multiple different rifles and calibers have been far too good to believe it is merely chance.

For a given length barrel, if you know the frequency at which it vibrates, it would be tempting to think you could simply subtract one OBT from an adjacent node in the Chris Long's OBT Node Table and determine whether the resulting difference was anywhere close to 1/2 the frequency, or the time it would take to move from the top to the bottom of its arc of travel. Unfortunately, measuring barrel frequency takes specialized equipment not owned by most shooters and the system is far more complex than that.

For example, the frequency of 1st order (Mode 1) harmonic motion at the muzzle analogous to the end of a model cantilever beam is relatively slow, perhaps on the order of 50 to 75 Hz, depending on the barrel and load velocity. That means the time it it would takes the muzzle to go from the top of its arc to the bottom would be around 13 to 20 ms (i.e. 1/frequency). Optimal barrel times would therefore need to be much longer than they actually are reported in the Table, more on the order of 10 to 20 ms, for Mode 1 to be the target. So the explanation of bullets exiting at the top of the barrel arc for one OBT node versus the bottom of the arc for the next OBT node is too simplistic and does not fit with actual measured data. In fact, the frequency of Mode 3 (around 1000 Hz) represents a more likely way in which optimized barrel times could mesh with barrel harmonics.

Another thing to remember is that OBT Nodes as listed in Chris Long's Table are not the only "accuracy" nodes. In fact, they are spaced pretty far apart based strictly on the values in the Table. In other words, velocity from a given barrel length must be increased/decreased substantially in order to hit the next adjacent OBT Node in the Table. If you use a ladder or some other form of load development testing that clearly reveal accuracy nodes, you will find that there are clearly other nodes in-between the values of the OBT Table. In terms of barrel occupancy time, these other nodes are clearly not as far from a given OBT Node as the OBT Node itself is from the next OBT Node.

So what does all that actually mean? To be honest, I have spent some time in the past going partway down that rabbit hole, but ultimately refused to go any further. There are many resources available to you if you wish to pursue some unifying theory between barrel harmonics and the concept of OBT. Personally, I find it much easier to load up a test series, go out and shoot them, and then let the target tell me where the rifle wants to tune in. I'm basically describing the difference between empirical and theoretical information. There are certain things we can readily do and obtain useful information from, without necessarily understanding the mechanism of how they work in exquisite detail. So you can easily develop a load that shoots extremely well without using any reference to barrel occupancy time, then go back later using QuickLoad and determine for yourself how well the barrel occupancy time of that load matches some value in the OBT Table. If you do that exercise a few times, it ought to be enough to convince one way or the other whether OBT Theory works in your hands. Personally, I very much like to understand WHY things work the way they do, especially with regard to ballistics, but I often keep coming back to a very simple idea that usually gets me where I need to be:

View attachment 1164739

You are correct on many of your assertions. For example, yes, you have to know the distance(or frequency) from one half node to the next. And yes, there are multiple frequencies happening at the same time, but a tuner is adjusting for the average of all of them, being at or beyond the muzzle. It is as simple as changing the frequency proportionately to in bore time. If you assume that you need to make a large adjustment in terms of amount of tuner travel, you're off to a bad start. The key to using a tuner is and always has been to be very, very methodical. If you move it randomly, you'll get random results. Yes, fwiw, I have invested in and actually measured these things using pretty sophisticated equipment..bot in a lab and range environment.
Tuners are not voodoo or black magic..not by a long shot. The results are calculable and predictable but the info is only as reliable as what you put into it. I've said it many times but I'll say it again...please move my tuner one mark at a time. On my standard tuner, that is .001" of tuner travel per mark. The results will speak for themselves and I saw that even before investing in measuring it. It was apparent on the target and the testing just verified it...I did still learn a ton from the testing but as far as using one...nope., already had seen that and so will anyone that does the same thing. They are very simple in use and really not that difficult to understand how they work, although, I'm not sure it matters much.

 

[Ned Ludd]

You are correct on many of your assertions. For example, yes, you have to know the distance(or frequency) from one half node to the next. And yes, there are multiple frequencies happening at the same time, but a tuner is adjusting for the average of all of them, being at or beyond the muzzle. It is as simple as changing the frequency proportionately to in bore time. If you assume that you need to make a large adjustment in terms of amount of tuner travel, you're off to a bad start. The key to using a tuner is and always has been to be very, very methodical. If you move it randomly, you'll get random results. Yes, fwiw, I have invested in and actually measured these things using pretty sophisticated equipment..bot in a lab and range environment.
Tuners are not voodoo or black magic..not by a long shot. The results are calculable and predictable but the info is only as reliable as what you put into it. I've said it many times but I'll say it again...please move my tuner one mark at a time. On my standard tuner, that is .001" of tuner travel per mark. The results will speak for themselves and I saw that even before investing in measuring it. It was apparent on the target and the testing just verified it...I did still learn a ton from the testing but as far as using one...nope., already had seen that and so will anyone that does the same thing. They are very simple in use and really not that difficult to understand how they work, although, I'm not sure it matters much.

The OP asked whether adjacent optimal barrel times represented the muzzle being at the top or bottom of its arc. I was attempting to provide my thoughts on the question. However, I never mentioned the use of a tuner, nor did I intend to, so I'm not at all clear why you quoted my earlier response in conjunction with a discussion about tuners. If you know something specific about a relationship between barrel tuners and optimal barrel time nodes, I'd be very interested in hearing more about it.

 

[gunsandgunsmithing]

The OP asked whether adjacent optimal barrel times represented the muzzle being at the top or bottom of its arc. I was attempting to provide my thoughts on the question. However, I never mentioned the use of a tuner, nor did I intend to, so I'm not at all clear why you quoted my earlier response in conjunction with a discussion about tuners. If you know something specific about a relationship between barrel tuners and optimal barrel time nodes, I'd be very interested in hearing more about it.

Touche.

 

[Zero333]

OBT is a hit and miss kinda thing. I've had about a 40% success rate in estimating accuracy nodes with QL.

Sometimes it's right on the money. Other times it's exactly where you'd think it wasn't.

 

[jvw2008]

@Ned Ludd

best summation of theory versus reality I’ve ever read on this subject!

Trust but verify - the target doesn’t lie.

 

[Straightshooter1]

- the target doesn’t lie.

It does when I'm shooting.

 

[jvw2008]

It does when I'm shooting.

Ya, that has been my excuse many times when I was chasing a node around the barn for the third time! Funny how we try to divorce our skill out of the total equation.

 

[243Mendoza]

OBT is not a measure of transverse vibrations since those vibrations depend on the barrel cross sectional inertia and longitudinal vibrations do not. So OBT is longitudinal vibrations.