OCW or Velocity Node?

[Glassaholic]

Found this article by 6.5 Guys while looking for information on OCW - http://www.65guys.com/10-round-load-development-ladder-test/

It got me to thinking, with long range we want low ES because the difference between high ES numbers could mean a miss in the field at longer distances. So loading to find that flat spot "a velocity flat spot where .4-.8 gr of powder doesn’t move the speedometer much" is really determining the best range for minimal differences between a range of charge weights. But help me understand, with OCW you're looking for accuracy variances between the different charge weights, but Scott is saying that he's found this accuracy sweet spot always lies within this velocity flat spot? Or is he saying we should fine the velocity flat spot and then adjust COAL from there to refine accuracy, but getting ES to a minimum is #1 priority? Not even sure I explained that adequately, but I'm always looking for better ways to skin the cat and this sounds like it might be just that.

 

[spife7980]

I tried it, it out me in the middle of a scatter node. I trashed it.

You need more than a few coincidental scatter points in my opinion.

 

[Glassaholic]

I tried it, it out me in the middle of a scatter node. I trashed it.

You need more than a few coincidental scatter points in my opinion.

I understand the idea of barrel harmonics and looking for the right bullet/speed combination with one particular barrel. I suppose Scott is saying that low ES is the priority, even if you get awesome 100yd groups but have crazy ES it's not a good load for long range. When you say scatter nodes are you talking about the ES or are you talking about groups?

 

[spife7980]

Groups, all seating depth tests were garbage in the middle of the charge weigh flat spot. 223 testing the new 53 varmageddon. 24.1-24.3 of xbr were my test points it pointed to being flat so I shot the seating depths at 24.2. Piling poop. Did an ocw and landed on 24.7 as my golden goose.

 

[Juggerxxx]

I don’t understand why there’s a difference between the two tests and you need to settle on one or the other. When I do an OCW I also clock bullet speed and use that to help me determine where the flattest speeds are, lowest SDs are, similar POI is and promising groups.

 

[vitalemj]

I tried it, it put me in the middle of a scatter node. I trashed it.

You need more than a few coincidental scatter points in my opinion.

Same here. Looking back on it, and in complete agreement with Spife7980, it seems possible to find a point where slight changes in powder charge are forgiving in the velocity department but that point also lands dead center of two OBT nodes or (if you don't buy into the OBT theory) happens to fall on an ultraviolent convergence point of frequency modulation; say verticle, horizontal and torsional vibration, causing the erratic grouping of the scatter node mentioned above. If your only watching for a flat spot in velocity how does that prove it is also a calm and forgiving point in the barrel harmonics cycle?

If you do the test and end up with both a flat spot in muzzle velocity and find an accuracy node it seems the recipe for the perfect load. Especially if as Scott says he is always striving to find that same load is safely just under the maximum pressure/muzzle velocity combination for that rifle. But how does finding a forgiving powder charge where velocity is concerned, prove you are at an optimal point of barrel modulation when the round exits?

 

[Glassaholic]

The 6.5 Guys even comment that this method has consistently worked for them. It almost seems that Scott's method is a means to quickly ascertain what is that sweet spot but that for some it doesn't work out so well. It seems like Scott's method is still a product of OCW so if it fails you're already on your way.

 

[supercorndogs]

Recent conversations with very learned mechanical
engineers are affording me some better understanding of
just why an OCW load works so well in the majority of
rifles chambered for the cartridge at hand.
Here are some of the major points coming to light:
Uniformity of velocity (meaning low extreme spreads of
velocity) are definitely not an indicator of the OCW
zone. We are actually finding that in many cases the
OCW zone does not have the tightest numbers--at least
initially. Fine tuning of the recipe with seating depth
variations and primer changes will improve the velocity
consistency, but simply shooting a succession of
graduated charges over the chronograph and looking for
a tight velocity spread will not lead you to the OCW.

It doesn't really give you any relevant info according to the person who wrote the theory. I don't use the chronograph until I am testing length.

 

[towerofpower93]

I tried the 6.5 guys velocity node test on a new 6.5x47 Lapua build and it did end up giving me two nodes which were further investigated with (2) 5 shot groups and velocity data from the whole 10rds. Both nodes were very accurate and I ended up settling on the higher node.

The above not withstanding, I also tried it in a short barreled AR-10 in .308 and got similar results to Spife. Found one or two velocity sweet spots with three different bullets and one powder, but none were winners in group size or ES/SD when I brought out a 10rd lot and put (2) 5rd groups together. I believe these poor findings were due to me using brass which would not be up to snuff for long range work, but was instead purchased as once fired and prepped on my Dillon 650 for 2 gun/3 gun matches. The accuracy was enough for these goals, but not long range work.

As they mention in their article, having all of your components as uniform as possible going into the velocity node test is critical, and I believe my above findings support this.

It's only an N of 1 with a precision rifle and fully prepped brass made with long range in mind, but I will be trying it again next time I get a new precision rifle or want to try a new bullet in an existing rifle.

 

[CharlieNC]

If you want an award for the chrono then believe it. If you want an award for the target then believe it. Ladder and ocw tests reflect barrel harmonics as it oscillators vertically at several frequencies to provide compensation for velocity differences. I have never heard an explanation why this would translate to velocity stability. For me the target nodes have always reproduced.

 

[TXBear]

Tried it out in a Tikka T3 since the long distance ranges were shut down. Found two 'nodes'. Only problem was, they were two charges side by side. The next load below and above didn't 'fit'.
I'll go back when the longer ranges are open and try a true OCW test.

 

[Glassaholic]

For me the target nodes have always reproduced.

What do you mean by target nodes, are you referring to groups? I think the authors indicated that even though you might have a load that groups well at 100 but has horrible ES then at distance (say beyond 600 yards) you'll begin to see different vertical impacts which could cause misses.

 

[spife7980]

But if it never groups to begin with your amazing 10 shot es doesn’t matter a bit getting scattered all over the hillside.

 

[Glassaholic]

But if it never groups to begin with your amazing 10 shot es doesn’t matter a bit getting scattered all over the hillside.

I absolutely agree. I suppose the question is which method can help you get both good groups and low es. I’ve never tried Scott’s method but I understand the convenience if it works; however, even though the 6.5 Guys seem to think it works it sounds like the majority of Hiders who’ve tried it have had issues which gives me caution.

 

[MarkCO]

Several engineers have also used the 10 round ladder test as described in the linked video, and found it to work well. I have run it numerous times now, and then follow it with another 10 shot string bumping only by 0.1 grains. I have found it to be repeatable and consistent in that I get small groups, and consistent data.

IF you can measure to the kernel, maybe there are other methods that will work as well or better.

 

[Kopfjager1]

I have used it as well but used 3 rounds per charge weight initially, then once I found a velocity flat spot I switched to 5 rounds. Ended up having to switch bullets because I could not get my groups with single digit SD/ES under 3/4 MOA. All my groups that were 1/4 MOA had SDs of 15-20 fps. Once I switched from ELD-Ms to RDFs, went back to the velocity flat spot and all my 5 round groups were all 1/2 MOA and under just chose the lowest ES and best group then messed with seating depth. Which my original seat depth was the most accurate and had the lowest ES. Did (4) 5 round groups to confirm ES and accuracy them went out to collect data all while shooting over the Magnetospeed V3 with the MKM adapter.

 

[CharlieNC]

What do you mean by target nodes, are you referring to groups? I think the authors indicated that even though you might have a load that groups well at 100 but has horrible ES then at distance (say beyond 600 yards) you'll begin to see different vertical impacts which could cause misses.

Read up on Audette ladder and OCW testing. These do not define a node based on groups. You are looking for the charge region where differences in weight have little to no impact on verticle POI, which basically maps out the harmonic movement of the barrel. In fact it is not unusual to see a series of shots to progressively impact lower as the charge weight increases. Some shoot load development at 100yd but I find much more definition at 200yd to see the node. I used to shoot these over the chrono, and it does not reflect the target node. Logically how can you depend on 1 or 2 shots with a chrono to reliably show a node, when at any given shot you don't know the es or sd? Believe the target.

 

[MarkCO]

When you try it and understand the science behind it, it really becomes quite simple and elegant. As much as I enjoy reloading and tinkering with loads and the corresponding data, I'd rather put 20 rounds downrange and have a great load as opposed to 100+ rounds of less consistent searching.

 

[_Raining]

When you try it and understand the science behind it, it really becomes quite simple and elegant. As much as I enjoy reloading and tinkering with loads and the corresponding data, I'd rather put 20 rounds downrange and have a great load as opposed to 100+ rounds of less consistent searching.

What's the confidence interval on that science of yours?

 

[ZiaHunter]

Having not heard or read about Scott's method I am not in a position to comment on it. I will chrono each round while doing a ladder test to identify the charge weight that provides the best group and lowest SD then fine tune from that point. Generally 5 rounds for each charge weight fired round robin. I have had great groups with poor SD's and vice a versa so focusing on one can lead you astray. With my F Class rifles I have seen changes in vertical dispersion with as little as .1 grain variance in charge weight so once I identify the best node during the initial test, I will run a subsequent test with smaller increments around that node as well as play with seating depth. Most of my testing revolves around tuning a replacement barrel so I have a fairly good idea at what charge weight and MV the node should be.

 

[Dewey7271]

I've run Scott's method on a few hunting rifles (5)and the results have been good. Easy to find something that shoots well without burning a bunch of powder and bullets up.
I use a MagnetoSpeed which simplifies matters. From a few friends results the optical chronos do not seem to do as well with this. Not slamming them at all just an observation.
I do only use flat spots of three "flat readings" or more as it seemed to me a 2 spot reading could simply be a normal variation of one at the high end of the variation and the second at its low end making it appear to be flat . If I don't get three then I don't use that area.

 

[towerofpower93]

I use a Labradar now and agree that a 'node' is only found with three or four charges which give a small total difference in velocity, with one or more in the middle flipping under the charge before it, really being the indicator for a velocity based node being found.

At least that's how my 6.5x47L nodes were found....

 

[isofahunter]

I shoot a ladder. Today I shot a new 308 load 168g TMK over Varget. Only 5 shots fired at 428 yards. Lightest charge was about 2 inches high of POA. 2,3,4 charges were 1 inch high of POA a with a total vertical dispersion of. 823 inches and charge 3 and 4 were .26 inches difference charge 5 was 4 inches high of POA.

Next outing I am loading 5 of 3,4 and another 5 splitting the two. 3 5 shot groups and the tightest one wins. Then load the 980 remaining TMK and get to shootimy.

IMO unless you are shooting benchrest time is better spent behind the gun then at the bench looking for a 2 SD load and .25 MOA groups.

 

[Clearlight]

I never had a chrono for a long time , was just using a friends occasionally to get
velocity if I couldn’t reverse engineer it . Sequence was always , pressure test ,
then powder charge for group , then tweak seating , then maybe re check
powder . If was grouping well at LR , I gave little mind to velocity outside of a
number to feed the solver . Using a Labradar on a weekly , if not sometimes
daily basis has added another layer of data , but I’m not convinced I’m much
better off group size wise . One hole and a great ES is confidence inspiring though .

 

[Darkside-Six]

I don’t understand why there’s a difference between the two tests and you need to settle on one or the other. When I do an OCW I also clock bullet speed and use that to help me determine where the flattest speeds are, lowest SDs are, similar POI is and promising groups.

Exactly! If you do a proper load workup with an OCW your node is usually in the middle of a flat spot. Scotts way is just a quicker way to find it with using less ammo.

i've started just shooting 3 round groups in .3 grain increments and measuring speed and SD. there is usually a spot of 2 loads where the velocity is almost identical. maybe 4-6 fps difference and thats is where i've been finding my loads.

 

[Rthur]

I like to use the ladder test to find the nodes.
Then use the ocw to fine tune seating depth.
Verify at range and good to go.

R

 

[Brosome]

I do the same, 3 round groups over a chrono for velocities and have had excellent results so far plotting them on a scatter plot helps visualize the data. You can also look at the groups like an OCW to confirm what the velocities are telling you and make sure the group is decent.

 

[rymart]

Ladder results never agreed with OCW results for me. Sometimes close, but not quite. I began to see a pattern...

Many 1000 yard bench rest shooters don't even own a chronograph. Those that do rarely use it for load tuning and have found that loads with extremely low ES/SD often don't print groups with very tight vertical.

A well tuned load for 1000 yard bench rest will consistently print tighter vertical than is theoretically possible based on its velocity spread...

Two words:

Positive Compensation

 

[trob_205]

Ladder results never agreed with OCW results for me. Sometimes close, but not quite. I began to see a pattern...

Many 1000 yard bench rest shooters don't even own a chronograph. Those that do rarely use it for load tuning and have found that loads with extremely low ES/SD often don't print groups with very tight vertical.

A well tuned load for 1000 yard bench rest will consistently print tighter vertical than is theoretically possible based on its velocity spread...

Two words:

Positive Compensation

Started from the top and was wondering when someone would bring up positive compensation. Took me awhile to wrap my head around the concept but I come to find out it’s real lol. That same 1000 yard 1/4 MOA load might only be 3/4+ MOA @ 100. Most of the SDs are 9-15. Saterlee wouldn’t really accept those numbers if they were ES.

If you’re shooting BR at a specific distance then who cares about SD/ES. If you’re only shooting 1000 yard matches then who gives a shit if it’s only 3/4 MOA at 100.

Tactically I’d rather have lower ES/SD and have a load that shoots a predicable 1/2 MOA all the way out than a rifle that shoots in the 1s and 2s at a grand or 600 and 3/4+ everywhere else.

 

[Jimgal62]

Started from the top and was wondering when someone would bring up positive compensation. Took me awhile to wrap my head around the concept but I come to find out it’s real lol. That same 1000 yard 1/4 MOA load might only be 3/4+ MOA @ 100. Most of the SDs are 9-15. Saterlee wouldn’t really accept those numbers if they were ES.

If you’re shooting BR at a specific distance then who cares about SD/ES. If you’re only shooting 1000 yard matches then who gives a shit if it’s only 3/4 MOA at 100.

Tactically I’d rather have lower ES/SD and have a load that shoots a predicable 1/2 MOA all the way out than a rifle that shoots in the 1s and 2s at a grand or 600 and 3/4+ everywhere else.

 

[Jimgal62]

I have used scott’s method for my 308 Win and 6.5 CM rifles. It works for me using 0.2 grain increments and having 3-4 charges defines a flat spot or nodal point.

Typically I find three nodes in my barrels and pick the highest or next highest node.

Then perform seating depth and stretch out to 500 or 600 yards in 10 shot groups to confirm precision.

 

[BFuller]

Read this through. Positive compensation makes sense in terms of an explanation for certain loads having less vertical dispersion base on barrel movement.

However, I don't understand what the physical mechanism is that would result in a non-linear relationship between charge weight and velocity at the muzzle. Has anyone heard a convincing explanation of what causes the formation of these "nodes" or "flat spots"; where adding energy doesn't result in an increase of imparted momentum?

 

[Ledzep]

Read this through. Positive compensation makes sense in terms of an explanation for certain loads having less vertical dispersion base on barrel movement.

However, I don't understand what the physical mechanism is that would result in a non-linear relationship between charge weight and velocity at the muzzle. Has anyone heard a convincing explanation of what causes the formation of these "nodes" or "flat spots"; where adding energy doesn't result in an increase of imparted momentum?

Yes, Small sample size.

Everyone will agree that your final load has at least 25-50fps of ES, no? So for every velocity reading you get, you landed in a +/- 15-25fps window from the actual true average velocity produced by that load.

I can't speak for positive compensation beyond the fact that if you shoot enough shots into a group, I've never had one grossly more horizontal than vertical. In other words at 200yd by the time you hit 20-30 rounds, pretty much every group is circular. By the time you get to 50, it's a circle. The distribution of shots within a group is, as far as I can tell, entirely random. Faster shots don't go low, slower shots don't go high. Once set on course, bullets don't know where to go, and don't correct for positive compensation on their own. In theory I can understand how it would be physically possible, in practice I have never found any evidence to suggest that A) you can create positive compensation, and B) that altering powder charge or seating depth has ANY control on the subject whatsoever. If someone has proof to the contrary I'm all ears.