Half nodes?

[Alex Wheeler]

It probably wouldn't be necessary to take hyper-accurate measurements regarding apparent changes in barrel length with different tuner settings. All that would be needed would be to analyze the barrel harmonics at different tuner settings. The effects of tuners at the correct setting on barrel harmonics have been fairly well-characterized and should be reproducible simply by monitoring the barrel harmonics. With the correct equipment, it might even be possible to do this by simply tapping the barrel with a mallet (or something similar) to set up the harmonics. This is not my idea, but was recently suggested to me by Steve Blair. However, a number of recent posts here regarding the use of tuners and my own reading on the topic suggests it is a reasonable hypothesis. At the very least, with the correct equipment, it would be pretty easy to test whether optimal tuner settings "predicted" by this approach matched, or were even close, to optimal tuner settings determined empirically through firing and group analysis. To be honest, I'm not sure exactly what is currently considered to be the optimal equipment necessary to do barrel harmonic analysis. However, I'm pretty sure that whatever it is, most people don't own one, which is likely one of the reasons people don't do it this way.

The target is the optimal equipment. That was the point I was trying to make.

 

[Ned Ludd]

The target is the optimal equipment. That was the point I was trying to make.

I agree completely. The target is the ultimate readout of load development. The point I was making is that taking advantage of all tools available to simplify and expedite the reloading process, including computer programs such as Quickload, can be a benefit and help save time, effort, and $$$.

 

[Rex Johnson]

I wanted to comment here. I have a 1941 Winchester Model 70 in .30-06 that does not behave as predicted by OBT. It's happiest load is 58gr IMR 4350 with a 165gr Sierra TGK-Gamechanger. It has a 24in barrel (24.125 to bolt face) and based on velocity measurements it defies both QL predictions and is not any multiple of an OBT node. I am sure that this is because the forend is pressure-point bedded, and QL is expecting a free-floated barrel. Thoughts?

P.S...Not rebarreling or free floating...This is a gem the way it is; I'm not going to change a thing.

 

[6ShotsOr5?]

For those like me that didn’t get the memo yet, I think this could be the background info needed to understand this thread:

“http://www.the-long-family.com/optimal%20barrel%20time.htm”

We’re having our family Thanksgiving today so I will have to catch up on this thread later.

Happy Thanksgiving everyone.

 

[Davey Boy]

Just trying to add a bit here. GRTools has an OBT calculator in it, and it computes the half-nodes for any barrel length using the load specs you have chosen. The full nodes line up pretty well with QL nodes.
Just my 2 cents. Try it it's free, just have to register. GRTools.de very similar to QL and even has a few more features.

 

[paulT]

Actually the whole purpose of tuning a barrel to a bullet velocity is to compensate for variations in muzzle velocity. A barrel moving upward in the vibration cycle is what we are looking for. We are not looking for a stopped barrel. There are some in the rimfire world that think a barrel tuner when properly set up "stops" the barrel, and once tuned the gun can shoot any ammo velocity at any distance and the barrel will still be tuned. Not true. Barrel tuning in a rimfire by changing the tuner weight and location to change the vibration frequency of the barrel is just the reverse of having a centerfire barrel with a fixed vibration frequency and changing the velocity of the bullet.

A fast bullet will exit sooner in the vibration cycle, while a slow bullet exits later. If you use an average target velocity that gets to the muzzle when the barrel is on the upward vibration cycle a fast bullet exits with the barrel pointing lower. A slower bullet exits with the barrel pointing higher. If you choose the right part of the upward cycle the normal velocity variation from bullet to bullet at the same load will be compensated for, and the vertical variation reduced.

As far as where the exact right spot may be could be (just my theory) at the 1/3 up position (velocity), and then another similar one at 2/3 up. Right at 50% up where vibration vertical velocity of the barrel is at a maximum, may be too much. This is probably where you see a dip in POI increase with velocity increase. The ideal spots may be just before that or just after.

So getting back to the OP's question, yes there may be two "sweet spots" fairly close together separated by an area where there is some over correction. But remember, you have to go all the way around the clock with increasing velocity to get to those spots again. So if we categorize the vibration cycle crudely into a clock starting at 12:00 you will have a bad area which peaks at really bad at 3:00 and goes to 6:00. Somewhere between 6:00 and 9:00 there will be a sweet spot, then an over correction zone at 9:00, followed by another sweet spot between 9:00 and 12:00. In some cases there may not be a dip or it is very minor at 9:00, and there is quite a wide sweet spot.

At least that is the way I see it. As far as the OBT, OCW, I remain skeptical. Ladder works if you have an accurate gun and do the test properly. Hard to do with a scatter gun though.

As far as where the exact right spot may be could be (just my theory) at the 1/3 up position (velocity), and then another similar one at 2/3 up. Right at 50% up where vibration vertical velocity of the barrel is at a maximum, may be too much. This is probably where you see a dip in POI increase with velocity increase. The ideal spots may be just before that or just after. Does this process show when conducting a ladder test in say .2 grain increments where the ladder and velocity climbs to a point where the bullet impact drops and velocity increase stalls or drops (optimum area 1) then next shot impact and velocity jumps ( optimum area 2) however would a few shots forming a flat spot on ladder at same horizontal be required to confirm possible sweet spot.

 

[Ned Ludd]

Just trying to add a bit here. GRTools has an OBT calculator in it, and it computes the half-nodes for any barrel length using the load specs you have chosen. The full nodes line up pretty well with QL nodes.
Just my 2 cents. Try it it's free, just have to register. GRTools.de very similar to QL and even has a few more features.

Davey - just out of curiosity, do the "half-nodes" appear to fall at a uniform distance in terms of barrel occupancy time between OBT nodes, whether it be equidistant between them, or closer to one than the next closest? Or are they somewhat more random as to where they are predicted to fall in between the other nodes? In other words, can ALL of these nodes be predicted to some degree of consistency with barrel time?

 

[6ShotsOr5?]

I saw that @Ned Ludd already posted the link to Chris Long’s paper when the thread first started 4=years ago. I just read The OBT paper and the theory makes a lot of sense to me. I’m retiring soon and buying QL is at the top of my list, right after buying my own computer since I have to turn in my company laptop. After I play with it and try things out I’m gonna come back and read this thread and hopefully it will all start to click. This is really interesting stuff and thanks to everybody that contributed to this thread.

 

[Davey Boy]

I started at node 7 and went up and down from that . 22.125" Barrel.

Nd ms chrg.

5.5 1.1807 41.28
6.0 1.2084 40.63
6.5 1.2560 39.69
7.0 1.3228 38.58
7.5 1.3702 37.35
8.0 1.3987 36.61
8.5 1.4463 35.44


Ned these are from just a load that I was trying. Calculations are from node 5.5 to 8.5 with the different OBT times and charge weights. The full nodes match up pretty much with QL's nodes. GRTools computes these 1/2 nodes which is handy. I haven't tried any of these calculations on targets yet as I just started using GRTools. I do compare to QL's nodes and calculations to see if they are similar and they seem to be.
As too your question, no they don't seem to be a consistant percentage apart. At least I don't think so.
If a person could get a hold of Chris Long's equation or algorithms to compute these OBT's maybe one could see a pattern. I'm sure there must be one, but that is beyond my pay grade.

Anyway do try the program it's free, and has been updated once since I started playing with it. Similar to QL but has some more features than QL.

 

[Davey Boy]

MASTER MODEL​
	​	​
	N Odd​	N Even​
	​	​
A​	4.42642857E-03​	4.40803571E-03​
B​	2.84942857E-02​	2.68380952E-02​
C​	-3.18785714E-03​	-2.40148810E-03​
D​	2.91180952E-02​	4.39015873E-02​
	OBT = (A*N + B)*L + C*N + D​
	N is node number, may be zero or negative
	L is barrel length in inches
	OBT is in mS

As an example, if the barrel is 24 inches long, and you desire to find the OBT for node 4, the equation would look like:

OBT = (4.40803571E-03 * 4 + 2.68380952E-02)*24.0 + (-2.40148810E-03 * 4) + 4.39015873E-02 = 1.101 …



Here is Chris Long's formula. This kind of math is beyond me. Maybe some mathmeticians out there can get it to compute half-nodes. I wouldn't know where to start.

 

[Ned Ludd]

I started at node 7 and went up and down from that . 22.125" Barrel.

Nd ms chrg.

5.5 1.1807 41.28
6.0 1.2084 40.63
6.5 1.2560 39.69
7.0 1.3228 38.58
7.5 1.3702 37.35
8.0 1.3987 36.61
8.5 1.4463 35.44


Ned these are from just a load that I was trying. Calculations are from node 5.5 to 8.5 with the different OBT times and charge weights. The full nodes match up pretty much with QL's nodes. GRTools computes these 1/2 nodes which is handy. I haven't tried any of these calculations on targets yet as I just started using GRTools. I do compare to QL's nodes and calculations to see if they are similar and they seem to be.
As too your question, no they don't seem to be a consistant percentage apart. At least I don't think so.
If a person could get a hold of Chris Long's equation or algorithms to compute these OBT's maybe one could see a pattern. I'm sure there must be one, but that is beyond my pay grade.

Anyway do try the program it's free, and has been updated once since I started playing with it. Similar to QL but has some more features than QL.

Thanks. I have developed a number of loads based on OBT Node 4, which is often attainable with the 30" barrels and many of the bullet weights I use. For the 200+ gr .30 cal bullets, I don't want to run the pressures necessary to hit Node 4 (i.e. poor brass life), and I don't want to deal with potential gun handling issues due to the increased recoil for an increase in velocity of only about 75 fps. So those loads are definitely "in-between" nodes that fall somewhere in between Node 4 and Node 5. I was just curious how predictable the in-between nodes might be with respect to barrel occupancy time as output by QL or GRTools. If they move around a bit with different cartridges or bullets weights, barrel time outputs are probably not the greatest approach to finding them. In my hands, the predicted OBT nodes seem to be very close to where they're predicted to be, so using barrel time outputs from QL can facilitate/expedite the load development process. I would have semi-guessed that the half nodes should follow suit, but I've never really tried to test it empirically.

 

[Davey Boy]

I can't answer the question of whether the half nodes are spot on or not. In my gun the QL full nodes are pretty good. A great place to start then fine tune. If you look at some of the nodes I posted they are about 3% apart + or - a bit. I once read that the nodes for a .308 were 3% apart. Think it was Newberry on his OCW theory. Those numbers are very close to that.

 

[VA_XTC_Shooter]

I have been working with GRT and I am having trouble with the nodes. I initially plugged in a load in GRT for my 6BR with AMAX 105s and Varget. I revised the case capacity, case length, bullet length and COAL. I ran the numbers based on an initial guess of 29.2 gr.

According to GRT, the 4-1/2 node was 23.96 gr with a velocity of 2808 fps. Node 5 was 28.18 gr. I was hoping to have the nodes reflect on target so I loaded up rounds in 0.2 gr increments from 29.4 down to 28.2 and shot 5-shot groups round robin.

All of the groups were mediocre except 29.4, it was fantastic. The real velocity average was 2848 instead of the predicted 2808. So when I got home and ran the OBT test with the observed velocities, it changed the node 4.5 back down to 28.1 gr, which was an AWFUL group.

I know this is a sample size of one, but right now I don't have much faith in the OBT theory. Everything is based on a set time for a barrel length. Pressures, powders, nothing changes the target OBT. To me, that just doesn't make sense.

Thoughts?