Looking For Information on Accuracy Nodes

[lokent]

I have some very basic questions to help me with my load development. For a cartridge similar to 7mm SAUM shooting a heavy for caliber bullet with a relatively small bearing surface such as a 7mm 180 Grain Berger VLD and typical match barrel of 26-30 inches, please help me with the following questions:

1) How many usable accuracy nodes would one expect?
2) How wide are typical accuracy nodes in terms of powder charge?
3) If one increases or decreases a powder charge within the "width" of an accuracy node, does the decrease in vertical come from a more consistent muzzle velocity or barrel harmonics or some other factor? In other words, should I expect similar muzzle velocities from all the different charges within an accuracy node or will barrel harmonics cause a bullet with a greater muzzle velocity have the same point of impact as a slower bullet within the accuracy node?

Thanks for the help!

 

[Heman]

Don't have experience with the 7mm RSAUM but with my .280 usually the nodes seem to have a 1gr spread on them with the middle being the sweet spot. With the 175gr SMK there is one at 2920fps, and there is another at 2840fps. However I found one at 2880fps too. Basically just boils down to the harmonics of the barrel.

On your 3rd question you really need to do a ladder test to get that answer. When I finally started doing them I do get a faster bullet that will shoot lower than one with less charge due to the barrel harmonics.

 

[lokent]

Thanks Heman! that is exactly the kind of information I was looking for. Was confused when people advocate shooting over a chrono during ladder testing seeming to imply that you are looking for a level muzzle velocity with increased powder charge. I was under the impression that when you don't get a muzzle velocity increase with increased powder charge, you are likely over-pressure.

Based on Hodgdon data, H4831 and 175gr bullets yield about 38.3 fps of muzzle velocity for each grain of powder. So I suspect a typical node to be about 40 fps or so wide. Is that about right?

I agree that you would need to be in the middle of the node with a cartridge having an extreme spread of less than 38 or you will be outside of the node with the extreme muzzle velocity shots. I guess if you can get your extreme spread under 20 or so you have a little more wiggle room.

Thanks again!

 

[Win94ae]

Was confused when people advocate shooting over a chrono during ladder testing seeming to imply that you are looking for a level muzzle velocity with increased powder charge.

I load develop using the Audette Ladder Test, and one of the patterns I look for is a leveling out of the velocity over so many .1 of a grain. If I don't get that leveling, there is no node.

The other pattern is a tightening of the POIs.

Some people can only load develop at 100 yards, the chronograph is most useful in that situation, because one can have an accuracy node with varying velocity; in which at extended ranges vertical stringing will be present.
The chronograph will revel that condition.

So I suspect a typical node to be about 40 fps or so wide. Is that about right?

Think of it more as in how many .1 of a grain the node is wide.
There are a series of vibrations contributing to barrel harmonics, there is no way to know how your bore will react.
With my 30-06s I usually can find 4 to 6 nodes within suggested charge weights; nodes ranging from .6 to 1.2 grains.

I guess if you can get your extreme spread under 20 or so you have a little more wiggle room.

Indeed! The proper powder for cartridge/bullet weight combination is key in achieving stable velocities.

 

[lokent]

A node 0.6 wide would equate to a velocity change of a little over 20 fps (assuming linear relationship between powder charge and muzzle velocity) with my bullet powder combo. I have often heard an extreme spread of less than 20 is good for long range match shooting.

How do you know the velocity leveling within such a small powder increment isn't due to measurement system error or random variation due to things like environmental fluctuations and/or barrel heat? It seems like it would be very difficult to meaningfully resolve such differences with an extreme spread of 10 to 20 without shooting a tremendous amount of rounds.

I've tried OCW and ladder tests and I have always been worried that my results are plagued with random fluctuations, shooter induced error and are not at all repeatable. I struggle greatly with how much replication is needed for meaningful results. I don't think I am alone here. Obviously competitive shooters that win are able to make this work for them. I am still struggling.

 

[jbpmidas]

I think experience helps here. When I completed my first ladder test, I didn't really know what I was looking for. I took my ladder target home and studied it and decided on a course of action from there. I have modified some from that first plan, but my way works for me.

 

[Win94ae]

How do you know the velocity leveling within such a small powder increment isn't due to measurement system error or random variation due to things like environmental fluctuations and/or barrel heat?

Do things to rule out such instances.

measurement system error: Don't ladder test on partly cloudy days. Use a spotting scope as to reduce the time it takes to note which shot hit where, (more frequent shots equate to more uniform conditions.) If you can't afford a spotting scope; color code your bullets with different color magic markers, the bullets will then mark the target.

barrel heat: Load your cartridge only right before you are ready to shoot.
If you experience different POIs as the barrel heats, re-bed your receiver to the stock.

It seems like it would be very difficult to meaningfully resolve such differences with an extreme spread of 10 to 20 without shooting a tremendous amount of rounds.

An Audette Ladder Test does not end with the completion of initial loaded cartridges. The nodes you find are potential loads, it is up to you how many cartridges you load with specific charge weights as you continue testing.

Get a data log book and record all your shots, even when not load developing; many serious marksman keep such records.

I've tried OCW and ladder tests and I have always been worried that my results are plagued with random fluctuations, shooter induced error and are not at all repeatable. I struggle greatly with how much replication is needed for meaningful results. I don't think I am alone here. Obviously competitive shooters that win are able to make this work for them. I am still struggling.

If you think your tests are plagued by "shooter induced error" get a gun vise or lead sled... or use that time to refine your shooting technique.