OBT and barrel length?

[Medium rare]

Has anyone tested whether the velocity node on a specific rifle changes as barrel length changes?

If I have a very long barrel and I am running in a slower node can I cut the barrel back and adjust load to same velocity and achieve same precision/accuracy? Or does the accuracy node shift with barrel length?

 

[Sheldon N]

According to the OBT theory, optimal barrel time changes as the barrel length changes and does not track exactly with velocity.

Calculating OBT generally requires both the OBT chart with the list of barrel times/nodes for each barrel length, as well as a copy of Quickload to use your specific load and gun data to determine what the barrel time actually is.

That's not to say you can't find a new node after cutting back the barrel, this is just the "formal" way of using the OBT theory.

 

[normmatzen]

Strange you should ask!

A fellow in Australia, Graham Mincham, wrote a book called,"Optimum Barrel Length." This book outlines how to find the lateral resonance of a barrel by shooting a ladder type series of 6, 5-shot groups with each group having a different powder charge. Then, using a program like "On Target," plot the group size vs charge. The ES curve shows the charge you should use where the ES is a minimum, and the group sizes indicate what the barrel length should be for proper vertical resonance.
I believe this book can be obtained from the South Australia Rifle Association.

 

[Medium rare]

According to the OBT theory, optimal barrel time changes as the barrel length changes and does not track exactly with velocity.

Calculating OBT generally requires both the OBT chart with the list of barrel times/nodes for each barrel length, as well as a copy of Quickload to use your specific load and gun data to determine what the barrel time actually is.

That's not to say you can't find a new node after cutting back the barrel, this is just the "formal" way of using the OBT theory.

So there is not a correlation that can be expected by just cutting the barrel. I cannot expect the node to shift up in velocity or down in velocity if barrel is cut in any predictable way?

 

[Medium rare]

Strange you should ask!

A fellow in Australia, Graham Mincham, wrote a book called,"Optimum Barrel Length." This book outlines how to find the lateral resonance of a barrel by shooting a ladder type series of 6, 5-shot groups with each group having a different powder charge. Then, using a program like "On Target," plot the group size vs charge. The ES curve shows the charge you should use where the ES is a minimum, and the group sizes indicate what the barrel length should be for proper vertical resonance.
I believe this book can be obtained from the South Australia Rifle Association.

That sounds like a great book. I need to get a copy. Thanks!

 

[Steve Blair]

So there is not a correlation that can be expected by just cutting the barrel. I cannot expect the node to shift up in velocity or down in velocity if barrel is cut in any predictable way?

A rule of thumb within a couple inches either way is:
If the barrel is shorter, pressure must increase to stay in the same OBT node.
If the barrel is longer, pressure must reduce to stay in the same OBT node.

 

[Sheldon N]

So there is not a correlation that can be expected by just cutting the barrel. I cannot expect the node to shift up in velocity or down in velocity if barrel is cut in any predictable way?

I hadn't looked into the cause/effects too much, but played around a bit in Quickload to see what the results would be.

Generally if you are working within the same OBT node and cut the barrel shorter, you will need to increase the powder charge to stay within the OBT node. Pressure goes up, barrel time goes down, and the net velocity may actually trend slightly higher the shorter the barrel. Velocity actually tracks the closest, but none of it is perfectly linear.

I do pay attention to OBT, but don't think that it's the be-all-end-all of load development. There are definitely good shooting nodes that are not OBT nodes, and just because it's at the OBT node doesn't guarantee that it will shoot amazing. My approach is to use OBT modeling to get a sense of what load is likely to shoot well, do an OCW style test around that node, then refine from there. I have found that the OCW nodes do match well with where the OBT theory predicted they would be. Not always exact, but close.

 

[Medium rare]

I hadn't looked into the cause/effects too much, but played around a bit in Quickload to see what the results would be.

Generally if you are working within the same OBT node and cut the barrel shorter, you will need to increase the powder charge to stay within the OBT node. Pressure goes up, barrel time goes down, and the net velocity may actually trend slightly higher the shorter the barrel. Velocity actually tracks the closest, but none of it is perfectly linear.

I do pay attention to OBT, but don't think that it's the be-all-end-all of load development. There are definitely good shooting nodes that are not OBT nodes, and just because it's at the OBT node doesn't guarantee that it will shoot amazing. My approach is to use OBT modeling to get a sense of what load is likely to shoot well, do an OCW style test around that node, then refine from there. I have found that the OCW nodes do match well with where the OBT theory predicted they would be. Not always exact, but close.

Thanks for the info. The reason I ask is I have a FTR gun that is shooting really well and I am building a new FTR rifle. I am running the slow node in my current rifle and I could get a similar velocity or faster with a shorter barrel in the new one.

 

[jlow]

A rule of thumb within a couple inches either way is:
If the barrel is shorter, pressure must increase to stay in the same OBT node.
If the barrel is longer, pressure must reduce to stay in the same OBT node.

I think this is correct because the concussion wave that travels up and down the barrel is traveling at a faster speed than the bullet. So first, since they are not traveling at the same speed, you have to adjust bullet speed if the barrel is shorter. Two, if the concussion wave is travelling faster than the bullet, if the barrel is shortened, the wave is going to reach the crown sooner, so you have to speed up the bullet to "catch up" and visa versa.

So a QL simulation. For my .223 with
- 20.313" barrel, my node 6 is 1.112 ms requires 24.22 gr of TAC and MV of 2,698fps
- 18.000" barrel, my node 6 is 0.989 ms requires 25.10 gr of TAC and MV of 2,714fps

 

[dmoran]

I think this is correct because the concussion wave that travels up and down the barrel is traveling at a faster speed than the bullet. So first, since they are not traveling at the same speed, you have to adjust bullet speed if the barrel is shorter. Two, if the concussion wave is travelling faster than the bullet, if the barrel is shortened, the wave is going to reach the crown sooner, so you have to speed up the bullet to "catch up" and visa versa.

So a QL simulation. For my .223 with
- 20.313" barrel, my node 6 is 1.112 ms requires 24.22 gr of TAC and MV of 2,698fps
- 18.000" barrel, my node 6 is 0.989 ms requires 25.10 gr of TAC and MV of 2,714fps

jlow -

The speed of sound in 416 Stainless Steel (commonly used barrel steel) is around 19,000-fps, and is what the pulse waves are computed by in Chris Long's OBT work.

Below is Chris' Pulse Reflection drawing:

The faster nodes are the ones just AFTER the pulse leaves the muzzle.
Those nodes are narrower in time, and often not as good as the ones that are just BEFORE the pulse arrives.
For example, a 27" barrel:
Node 3 = 1.147ms - just after pulse "faster"
Node 4 = 1.235ms - just before pulse "slower"
Node 5 = 1.380ms - just after pulse "faster"
Node 6 = 1.468ms - just before pulse "slower"

The "faster" nodes (3 & 5) are narrower and more difficult to get a load
developed, whereas the "slower" nodes (4 & 6) are broader and easier to tune for.

The wave reflection off the muzzle (just prior to node 3), is basically ringing a bell (the muzzle).
The ringing decreases with time, so node 4 is wider & more stable than node 3
There is less time between N3/N4 (.088ms) than N4/N5 (.145ms). Due to the longer pulse dwell time at the muzzle.

 

[Medium rare]

I think this is correct because the concussion wave that travels up and down the barrel is traveling at a faster speed than the bullet. So first, since they are not traveling at the same speed, you have to adjust bullet speed if the barrel is shorter. Two, if the concussion wave is travelling faster than the bullet, if the barrel is shortened, the wave is going to reach the crown sooner, so you have to speed up the bullet to "catch up" and visa versa.

So a QL simulation. For my .223 with
- 20.313" barrel, my node 6 is 1.112 ms requires 24.22 gr of TAC and MV of 2,698fps
- 18.000" barrel, my node 6 is 0.989 ms requires 25.10 gr of TAC and MV of 2,714fps

Thank you! This answered my question perfectly. Hopefully this holds true for 308s as well

 

[Medium rare]

jlow -

The speed of sound in 416 Stainless Steel (commonly used barrel steel) is around 19,000-fps, and is what the pulse waves are computed by in Chris Long's OBT work.

Below is Chris' Pulse Reflection drawing:

The faster nodes are the ones just AFTER the pulse leaves the muzzle.
Those nodes are narrower in time, and often not as good as the ones that are just BEFORE the pulse arrives.
For example, a 27" barrel:
Node 3 = 1.147ms - just after pulse "faster"
Node 4 = 1.235ms - just before pulse "slower"
Node 5 = 1.380ms - just after pulse "faster"
Node 6 = 1.468ms - just before pulse "slower"

The "faster" nodes (3 & 5) are narrower and more difficult to get a load
developed, whereas the "slower" nodes (4 & 6) are broader and easier to tune for.

The wave reflection off the muzzle (just prior to node 3), is basically ringing a bell (the muzzle).
The ringing decreases with time, so node 4 is wider & more stable than node 3
There is less time between N3/N4 (.088ms) than N4/N5 (.145ms). Due to the longer pulse dwell time at the muzzle.

This is some good info. I'm not sure if I can decipher it correctly. How would you know which node your load is in?

 

[savagedasher]

This is some good info. I'm not sure if I can decipher it correctly. How would you know which node your load is in?

Only paper will tell . Larry

 

[BoydAllen]

Once, some years back, I did a test that may speak to this issue in a round about way. I was shooting a relatively light bullet (62 gr.) in a PPC barrel, and had decided to see if I could improve its performance by steadily increasing charge weight. I was shooting a powder that likes a good bit of starting pressure, and had hypothesized that the rifling form (5 C) took less pressure to engrave the bullet, and therefore might need additional powder to reach the desired pressure. I found a good load a few tenths over 30 grains, but it was a pain to get that much powder in cases. At that point, I decided to see what would happen if I taped a weight flush with the muzzle of the barrel. I had one of the Possum Hollow trimmers in my reloading kit, the kind that indexes off of the case shoulder, and a roll of electrical tape. I put a length of tape on the bottom of the barrel, positioned the trimmer so that it was flush with the end of the barrel (at 6 O'clock) and wrapped it to the barrel with two layers of tightly stretched tape, so that it was secure. With the weight, the node required slightly less powder, which made loading easier, and when I cut the tape and removed the weight, it returned to where it had been before the weight had been added. If I remember correctly, the barrel would have been about 20.5" long, and was of typical benchrest contour. This might come in handy in a situation where, one has found a node that is a little slower than is desired, or perhaps the case is not as full as one would like at the heaviest accurate charge. By adding weight to the muzzle, one could change the vibration of the barrel so that the node just above would occur at a slightly lower charge weight that was within allowable pressure. So often we seem to believe that some sort of machining operation is needed to experiment with these sorts of things. While the taped on weight did not look like much, it worked. Perhaps some of you might want to do a little experiment of your own.

Back when tuner theory was being more commonly discussed, and Varmint Al had just published his findings, I asked him about the effect of thinning a barrel mid length. He ran the numbers and found that the effect was similar to lengthening the barrel or adding weight at the muzzle. My point is that there is a lot of work that could be done exploring the effects of different barrel contours, and that this exploration may be beneficial to the pursuit of improved average accuracy. Al's experiments showed that, for the rifle that he used as a model adding weight at the muzzle, increasing barrel length, or simply thinning the barrel in the middle, that some degree of positive compensation could be achieved, making uniformity of velocity within a given group less critical.

 

[jlow]

Dmoran - Thanks Donovan! As usual, some thought provoking information. First, I knew speed of sound propagation was fast but 19,000 fps was a bit of a surprise.

About the fast vs. slower nodes, do you know why some are narrower?

I am also wondering about the timing of the node i.e. “just AFTER the pulse leaves the muzzle” vs. “just BEFORE the pulse arrives”. I’ve always thought the nodes were related to when the pulses were furthest from the muzzle i.e. when the pulse get back to the chamber i.e. when the crown would be least disturbed?

Medium rare – as to which node your load is in? First, you generate barrel time using the Excel spread sheet which give you barrel time for each node for a specific barrel length. Then using QuickLoad (QL). When you put in your bullet, barrel length, cartridge, case volume, powder, powder charge, QL will tell you the barrel time for the load. Assuming you are centered in your load on a node, the barrel time QL gives you will correspond to one of the barrel time associated with a specific node number. If it is not centered, adjust powder weigh until it gives you a barrel time that is the same as the one reported in the spreadsheet - that's how you find an accuracy node with QL.

 

[jlow]

Once, some years back, I did a test that may speak to this issue in a round about way. I was shooting a relatively light bullet (62 gr.) in a PPC barrel, and had decided to see if I could improve its performance by steadily increasing charge weight. I was shooting a powder that likes a good bit of starting pressure, and had hypothesized that the rifling form (5 C) took less pressure to engrave the bullet, and therefore might need additional powder to reach the desired pressure. I found a good load a few tenths over 30 grains, but it was a pain to get that much powder in cases. At that point, I decided to see what would happen if I taped a weight flush with the muzzle of the barrel. I had one of the Possum Hollow trimmers in my reloading kit, the kind that indexes off of the case shoulder, and a roll of electrical tape. I put a length of tape on the bottom of the barrel, positioned the trimmer so that it was flush with the end of the barrel (at 6 O'clock) and wrapped it to the barrel with two layers of tightly stretched tape, so that it was secure. With the weight, the node required slightly less powder, which made loading easier, and when I cut the tape and removed the weight, it returned to where it had been before the weight had been added. If I remember correctly, the barrel would have been about 20.5" long, and was of typical benchrest contour. This might come in handy in a situation where, one has found a node that is a little slower than is desired, or perhaps the case is not as full as one would like at the heaviest accurate charge. By adding weight to the muzzle, one could change the vibration of the barrel so that the node just above would occur at a slightly lower charge weight that was within allowable pressure. So often we seem to believe that some sort of machining operation is needed to experiment with these sorts of things. While the taped on weight did not look like much, it worked. Perhaps some of you might want to do a little experiment of your own.

Back when tuner theory was being more commonly discussed, and Varmint Al had just published his findings, I asked him about the effect of thinning a barrel mid length. He ran the numbers and found that the effect was similar to lengthening the barrel or adding weight at the muzzle. My point is that there is a lot of work that could be done exploring the effects of different barrel contours, and that this exploration may be beneficial to the pursuit of improved average accuracy. Al's experiments showed that, for the rifle that he used as a model adding weight at the muzzle, increasing barrel length, or simply thinning the barrel in the middle, that some degree of positive compensation could be achieved, making uniformity of velocity within a given group less critical.

I must say that I am always a bit confused about this.

The question is when you tape the weigh to the end of the barrel, are you changing the location of the node or are you widening the node. I’ve always though it widening like if you are using a heavier barrel and was not changing the location since that barrel time Excel spreadsheet is affected only by length? On the other hand, a tuner that is put on the end of the barrel does not change barrel length either since barrel length is defined as distance from bolt fact to crow and a tuner does not extend the crown?

 

[BoydAllen]

I think that I need to clarify something. I was using node in the sense of tuning, the charge weight or range of charge weight that produced the smallest group. If I had to guess, it would be that the added weight lowered the barrel's resonant frequency, but that is just a guess. I think that it is more important to report what happened and how that information might be useful. In the case of my limited experiment, adding weight to the end of the barrel, slightly lowered the weight of the powder charge that was needed to achieve peak tune. There is plenty of experience that tells us that for rifles of this type, tuning node spacing is around 1 to 1.2 grains of powder, which makes the difference between completely out of tune, and fully in tune is .5 to .6 gr. The change that I saw was about half that.

 

[dmoran]

Boyd -
Interesting input, but not really on T with OBT.
Two seperate subjects with different effects - IMO.
Donovan

---------------------------------------------------------

jlow -
Simpliest way I can think of to explain it is: when the bell is ringing at one end, there would be echoing in the barrel and at the other end as well.
Look close at Chris' drawing. It tells the whole story well - IMO
Donovan

 

[jlow]

Thank you! This answered my question perfectly. Hopefully this holds true for 308s as well

Pretty sure the OBT theory holds true for all calibers and is not affected by barrel profile, just barrel length. QL works perfectly for my .308 loads.

 

[gunsandgunsmithing]

Is this what some of you are talking about in regard to tuning? Some of you may be familiar with a man named Tony Purdy. He's best known in the shooting community for introducing a tuning method called PRx tuning..or Purdy Prescription. The problem I have with it is that the air column is changing as the bullet travels down the bore. There are several rimfire br shooters that believe in it.