Choosing the “right” barrel length.

[Brians356]

I did some simulation using QL. ... So as you can see, with an 18” barrel, you really only have two nodes (6 & 7) that you can reach and the powder weights are either 42.3 grains or 45 grains.

In the interest of maximum clarity, could you define "node" WRT QuickLoad?

I suggest you read this review:

http://www.accurateshooter.com/gear-reviews/test-quickload-review/

Thanks. But there is not one occurrence of the word "node" in that review - unless my Firefox page search has a gaping hole in it.

A node is a powder weight that produce a tight group. With any caliber/bullet/powder/case/temperature/seating configuration, there are discrete powder weights that gives a tight group. We call them accuracy nodes.

What term is used on that page to refer to "node" then? (I'd like to see where QL is stipulating which charges will produce tight groups. That could save me a lot of time and components!)

New question: How does QL determine what powder weight will shoot accurately in a particular gun? Or, are you saying the so-called "accuracy nodes" are universal, i.e. for all guns shooting those components?

 

[jlow]

Not in a position to teach you how to use QL as it is pretty complicated, it is also not really my area of expertise, and beyond the scope of this thread.

In general, the accuracy node of any rifle is determined by its barrel (length and barrel time in QL), the powder, the caliber, temperature, the specific bullet, the case you are using (volume, length), and how you seat your bullet. You also cannot just plug numbers in and get there. A chronograph is required for you to get MVs to adjust QL so that the software is properly tailored to what you are using. If you are really interested, suggest you buy the software and read the manual and do research.

 

[Brians356]

Fair enough.

Anyone out there using QuickLoad to predict what powder charge(s) will be accurate in their rifle?

 

[mikecr]

My version of QL(3.6/DLL:1.00) does not predict nodes, accurate or otherwise. It does not show OBT in any sense. http://forum.accurateshooter.com/index.php?topic=3814598.0
It provides barrel time, and it provides 'what-if' loads matching in barrel time when selected. This might be applied external to QL any way you want, like in OBT calcs.
OBT is 'optimum barrel time'
http://the-long-family.com/optimal%20barrel%20time.htm

jlow might be matching barrel times with loads at different barrel lengths, or targeting specific barrel times with these variances -to reach OBT nodes.

 

[Brians356]

My version of QL(3.6/DLL:1.00) does not predict nodes, accurate or otherwise. It does not show OBT in any sense. http://forum.accurateshooter.com/index.php?topic=3814598.0
It provides barrel time, and it provides 'what-if' loads matching in barrel time when selected. This might be applied external to QL any way you want, like in OBT calcs.
OBT is 'optimum barrel time'
http://the-long-family.com/optimal%20barrel%20time.htm

jlow might be matching barrel times with loads at different barrel lengths, or targeting specific barrel times with these variances -to reach OBT nodes.

That's curious, as the chart that jlow posted (ostensibly as a QL product) contains the label "OBT" so apparently his version of QL does calculate OBT.

My hunch is that modeling OBT with software to predict accuracy nodes for a particular setup is probably about as reliable as modeling climate to predict future global temperatures. Chaotic multivariate nonlinear systems do not lend themselves to such simulations. Just a hunch - but a strong one.

 

[Steve Blair]

That's curious, as the chart that jlow posted (ostensibly as a QL product) contains the label "OBT" so apparently his version of QL does calculate OBT.

My hunch is that modeling OBT with software to predict accuracy nodes for a particular setup is probably about as reliable as modeling climate to predict future global temperatures. Chaotic multivariate nonlinear systems do not lend themselves to such simulations. Just a hunch - but a strong one.

[br]
No, that is a spreadsheet fragment, not QL. QL can be used to approximate OBT nodes. That does not mean that a load with a given barrel time will be accurate. QL is useful, in my experience, for predicting combinations that will likely produce stable, potentially accurate loads.

 

[Brians356]

No, that is a spreadsheet fragment, not QL.

Seems plausible, but I'll defer and wait for jlow's explanation of the source of his chart, as it was tacitly presented as a QL simulation (no reference to any external massaging or calculations.)

 

[Steve Blair]

Here's a reference chart I made using a spreadsheet that lists the discrete OBT values with an accompanying chart. This may make it easier the visualize the node relationships.

 

[jlow]

Nope, mikecr is correct. OBT does not come from QL but from that spreadsheet in the link he put up. What I show is just a spreadsheet I put together to keep track of data. QL was used to calculate barrel time to match OBT from the spreadsheet.

 

[Brians356]

Ok, got it. Thanks for your patience. I was not opening all the other links and spreadsheets, which would have tied this together for me.

Now I will take the time to study and digest this Grand Unification Theory of sorts.

"gstaylorg" (et al) if your OBT predictions continue to be confirmed by observation, perhaps someone can assemble a checklist listing the discrete steps of your process, something akin to Erik's load development checklist.

PS

I see the linked to white paper and ancillary material spells it out. However, the original white paper was last updated in 2004. So any updates, observations, or simplifications of the tables and methods posted here would be appreciated. If a more comprehensive thread on OBT theory and practice is out there, please point to it!

 

[Brians356]

In the OBT tables, how is barrel length measured? The white paper mentions measuring from the middle of the chamber, but nominal barrel length seems to what's been discussed here in this thread.

 

[mikecr]

When I messed with it (a long time ago), I had a spreadsheet generator to make tables that began from chamber center and then each successive wave travel was full barrel length(breech-muzzle-breech-etc).
I'd probably have to dig a 90s vintage laptop/disk out of the closet to find it..

 

[jlow]

QL manual on page 62 states that barrel length is "measured length from bolt face to muzzle".

 

[mikecr]

Ah, but the transient doesn't begin or end at the bolt face(nothing to do with barrel length), and an error here would compound with each cycle.

See the theory as applied holds some generalizations that make it both correlating or not. Just depends on much you want it one way or another.
This is why I abandoned the OBT rainbow chase.

 

[jlow]

It’s hard to say without knowing how the author of the program uses the information.

What you say could be correct, or another possibility could be that he knows that trying to have the user guess where the middle of the chamber is would be more of a problem than automatically factoring that into the calculations which he would have better control – that is how I would do it if I had a choice when writing the software.

 

[Brians356]

Well the question relates not to QL (directly) but to this pre-computed table:

http://the-long-family.com/OBT%20Table.pdf

It's a nice table, but unless I know what "barrel length" assumes, it's useless.

WRT QuickLoad, I assume "barrel time" uses nominal barrel length (to boltface), and the specific cartridge to determine actual bullet distance traveled to the muzzle.

 

[mikecr]

No, QL barrel time would be from ignition(T0) to muzzle.
It shows as '10% Pmax to muzzle'

 

[Brians356]

No, QL barrel time would be from ignition(T0) to muzzle.
It shows as '10% Pmax to muzzle'

Yes, but please read what I said. QL barrel time computation relies on a variable - barrel length. And QL barrel length may be different than the OBT table's barrel length.

When you enter "barrel length" into QL, I believe it is the nominal barrel length. My Rem 700 VS has a 26-inch barrel, so don't I enter "26" into QL as the barrel length (from which barrel time is computed)? That entered barrel length is to the bolt face, but the bullet does not travel that far, so QL must subtract the chamber length to arrive at the barrel length that the bullet travels, and uses that to calculate barrel time.

 

[mikecr]

From QL manual:
Barrel Length -> Input, measured length from bolt face to muzzle
Bullet Travel -> Output calculated from bullet base to muzzle
Barrel Time, 10%Pmax to muzzle -> Time from chamber pressure 10%Pmax to bullet base exiting muzzle

From this I consider Bullet Travel Time from chambered bullet base position to bullet base exiting muzzle. This is not 'barrel time' as 10%Pmax puts the bullet 1-2" down the bore.
Questionable(for me): where is the pressure peak manifesting within barrel(initiating disturbance point for OBT wave calc)? In other words, where is the analogous hammer hitting the barrel to begin? I could be considered near the bolt face(not at it), as this presents least chamber containment support, but I don't really know. At pressure peak the bullet will have traveled some, so the strike point would be between breech and bullet base at pressure peak.
That's a relatively big range.

From that initial point onward, speed of sound back & forth in barrel steel is easier, but could not actually apply to my barrel at it's individual shot/barrel temp, like it would apply to your conditions(unless same)..
Given this, I feel like there are too many variables to hang a hat on, and the OBT wave is only one mode of barrel change anyway. Add Varmint Al's whipping models, Vaughn's recoil models, and yet to be seen twisting models , and it all regresses to chaos.

 

[Brians356]

I tend to agree. It's seems far too complex to distill down to a two-dimensional array with barrel lengths along one axis and several OBTs along the other. But I suppose it could suggest some barrel times which, if one concentrated his efforts around, might speed the experimental discovery of an accuracy load. But it seems likely one of the more conventional ladder-type approaches might suggest a promising accuracy node just as quickly.