OBT using Gordon's Reloading Tool (GRT)

[vonb]

I've been using Gordon's Reloading Tool for the last 6 months. I'm amazed at the amount of data that the tool provides. I also really like the fact that the tool uses crowd sourcing to refine the data based on other's real data from a chrono, pressure trace, etc.

With that said, for those that use it, I have a question regarding Optimal Barrel Time within the tool. How many of you have found the OBT nodes to be close or spot on when you input YOUR specific information related to barrel length, twist length, case volume, cartridge length and brass length? For some of my rifles where I have known to have a solid load, the OBT was about spot on.

What has been your experience when testing the recommended OBT nodes?

 

[jelenko]

I've just started using it. At first I found GRT predicted 3 of my loads I had developed through ladder testing.
One of those, though, was an even half node which is 'dangerous' because it's very close in time to when the main pressure was arrives at the muzzle - just very good info.

The first new load that GRT predicted [again, after calibrating with known velocity for a specific load] didn't do so good => i.e., 3+ MOA groups The powder was MR2000 - which looks like there isn't a lot of data yet and GRT predicted mid 80% burn. So, I'm going to try a charge for a load with more powder and also confirm with known good loads that the barrel hasn't suddenly gone wonky.

Others on the US Rifle Team forum have reported good results.

 

[GuitarmanNick]

Just started using it recently and cannot help with an answer to your question.

I will say that some loads that I developed on my own seem to be performing very close to what the tool suggests. I have been side tracked the last week or so and need to enter a few more and compare results.

I am a novice at using this tool but it looks like it can be very helpful as long as the variables are all entered correctly.

 

[King Gizard]

I've been using GRT for about 4 months now & have had great success.
The 1st thing I did was to measure case volumes & all relevant data & input all the info for my cases.
I then compared my velocities to the GRT OBT & velocity & have been astonished at how closely all of my really good loads compare.
The best way I've found to use the program is to load at least 10 initial cartridges at CAOL, CBTO & bullet & powder type selection & get velocities to enter into the program. Once GRT has the velocities, it can adjust the powder burn rate to suit the measured results & give the necessary OBT velocity.
I've found that once GRT has measured velocities to work with, the OBT adjusted charge weights are pretty darn close & give a very good starting point that gets you very close to the OBT velocity. If the GRT charge weights aren't spot on, I adjust the powder weight to achieve the OBT velocity & just watch the groups shrink like a Johnson in ice water.
I wish I had come across this program earlier. It would have saved me a lot of time & expense.

 

[Ned Ludd]

In my hands using QuickLoad rather than GRT, optimized loads are generally just a tick slower than predicted in Chris Long's OBT table. For example, OBT Node 4 for a 30" barrel is 1.3684 ms. My best loads for several rifles with 30" barrels using bullet weights and powders that actually allow hitting Node 4 at safe operating pressures show QuickLoad Barrel Time outputs just a bit slower, typically around 1.370 to 1.376 ms, or so. That may be due to the way the QL Barrel Time output is generated, and the only way to know whether GRT is the same is to find an optimized load and obtain the barrel occupancy time from GRT and see how it compares to the value in the OBT table. Only over time and the development of multiple loads corresponding [in my opinion] to OBT Node 4 has it been possible for me to draw the conclusion that the trend with QuickLoad is that the predicted barrel time is typically a tad slower than listed in the OBT table.

You will find a wide variety of responses here to inquiries regarding OBT Nodes. Some swear by them, some not at all. I think the safest conclusion is that not everyone has found an apparent "OBT Node" for their setup exactly where it may be predicted to be based on Reloading software Barrel Time outputs. You still need to test across a "window" of barrel times spanning the predicted OBT Node to determine whether you think there may be something worth pursuing in that region. In that regard, perhaps it's better to think of the OBTs listed in the table as more of a rough guideline to a velocity/pressure/barrel time region that may be worth testing, rather than a highly detailed roadmap leading to an optimized charge weight. Certainly the OBT Nodes are not written in stone.

 

[King Gizard]

In my hands using QuickLoad rather than GRT, optimized loads are generally just a tick slower than predicted in Chris Long's OBT table. For example, OBT Node 4 for a 30" barrel is 1.3684 ms. My best loads for several rifles with 30" barrels using bullet weights and powders that actually allow hitting Node 4 at safe operating pressures show QuickLoad Barrel Time outputs just a bit slower, typically around 1.370 to 1.376 ms, or so. That may be due to the way the QL Barrel Time output is generated, and the only way to know whether GRT is the same is to find an optimized load and obtain the barrel occupancy time from GRT and see how it compares to the value in the OBT table. Only over time and the development of multiple loads corresponding [in my opinion] to OBT Node 4 has it been possible for me to draw the conclusion that the trend with QuickLoad is that the predicted barrel time is typically a tad slower than listed in the OBT table.

You will find a wide variety of responses here to inquiries regarding OBT Nodes. Some swear by them, some not at all. I think the safest conclusion is that not everyone has found an apparent "OBT Node" for their setup exactly where it may be predicted to be based on Reloading software Barrel Time outputs. You still need to test across a "window" of barrel times spanning the predicted OBT Node to determine whether you think there may be something worth pursuing in that region. In that regard, perhaps it's better to think of the OBTs listed in the table as more of a rough guideline to a velocity/pressure/barrel time region that may be worth testing, rather than a highly detailed roadmap leading to an optimized charge weight. Certainly the OBT Nodes are not written in stone.

I've spoken to a few guys about their results &, I think that those who have problems tend to be going off the powder charge weights instead of the muzzle velocity which dictates the OBT node.
As far as I can tell, the OBT nodes are based primarily on relatively fixed numbers ie; bullet acceleration & barrel length. The rest of the math is centred around obtaining the muzzle velocity from powder type, burn rate, etc, etc, etc. I've found that the OBT nodes are pretty much spot on with the projected charge weight varying to obtain the OBT node muzzle velocity. The OBT node muzzle velocity is based mostly on the barrel length & bullet acceleration rate so, the OBT nodes are really the fixed part of the load recipe with the powder charge being the variable in order to match a particular OBT muzzle velocity.

 

[dstoenner]

I’ve been working with GRT for about a year now. I’ve found the obt to be spot on for big cases like 308 and 30-06. It is close on my 6x47 lapua but dies on my 223 bolt or AR15. So bad i have abandoned using it to predict loads and gone back to ladders. I do use it to get a read on pressures for a given load. I did measure case capacity. Corrected bullet length to my batch of bullets but with small cases it just fails for me.

i think it has value but right now, for me, it isn’t OBT.

David

 

[jelenko]

I've been using GRT for about 10 months - only for 223 in an ar15. Have measured fired case volume and actual bullet lengths.
Note: Because my discipline only requires 0.5 to 0.7 MOA and because I test from prone with a sling, what I think is a 'node' could be off the actual node by a bit.

That said, I've found GRT to predict a node ~ 60% of the loads I've tried. Another 20% - 25%% are within 0.3 grains. In one of the instructional videos, the author says that, in his experience, GRT predicts nodes 60% of the time; 40% the node is not where GRT predicts.

I've had a couple cases where one of the nodes GRT predicts gives good results but the next node is no where close.

Similar to what others have said, I use GRT to find a load with the pressure and velocity I'd like. If the groups aren't satisfactory, I'll do a 'ladder' test with the predicted node in the center.

I've also started using GRT to identify the best powder for the bullets and barrel length I want to use.

 

[dstoenner]

I've also started using GRT to identify the best powder for the bullets and barrel length I want to use.

Now that my standard powders, Varget and H4350, aren’t available I’ve been using it to evaluate alternatives that i can get so i can reserve my standard powders for matches.

David

 

[JFrank]

What is new about OBT besides the description ?

(Nodes,) flat spots in volocity identified by chronographs, plateaus identified through ladder testing , overlapping of rounds when shot at the same POA etc. lack of vertical variances in horizontal poa, (ocw) are they not all results of optimized exit timing just described differently ?

 

[dstoenner]

Supposedly the mussle is at a null in its vibrations cause by the cartridge going bang.

david

 

[jelenko]

What is new about OBT besides the description ?

(Nodes,) flat spots in volocity identified by chronographs, plateaus identified through ladder testing , overlapping of rounds when shot at the same POA etc. lack of vertical variances in horizontal poa, (ocw) are they not all results of optimized exit timing just described differently ?

It's GRT that's new - nodes have always been there [Sort of a 'duh' statement].
Once GRT is calibrated to your load/rifle, it has a function to predict nodes based on the Optimum Barrel Time theory by Chris Long. To the extent it accurately predicts actual nodes, it can save time finding a load. I would assume that the more 'demanding' the accuracy requirements are, the more additional tuning would be needed.

At the same time, in other threads, several people [who, I think, compete in some form of benchrest] reported finding nodes between the ones predicted by Quickload or GRT.

 

[JFrank]

Should be a winner for those who like to collect data and predict loads.
Thanks

 

[Ned Ludd]

Should be a winner for those who like to collect data and predict loads.
Thanks

The use of reloading programs such as QuickLoad or GRT can help expedite the reloading process largely because of the barrel time output and the availability of Chris Long's OBT node barrel time table. It does not preclude testing, but can facilitate finding the best charge weight region for initial testing. Whether one buys into Chris Long's explanation of the mechanism by which the OBT nodes exist (i.e. longitudinal shockwave theory) or not, too many people use these nodes to deny that they exist. An important observation about the OBT nodes is that they are relatively far apart in terms of velocity/charge weight/pressure. There are clearly additional "accuracy nodes" that lie in between the predicted OBT nodes. In my hands, there is generally a noticeable difference between the non-OBT, or "in-between" nodes and the OBT nodes, with the OBT nodes typically being more generous (wider) and possibly having even a little better precision.

It can certainly be argued that the use of barrel time in load development is no different/better than using ladder or OCW testing procedures that don't necessarily require any velocity measurements in order to interpret the results. That is why a number of shooters that employ reloading software such as QuickLoad or GRT use it primarily as a guide to choose a starting charge weight region for testing, and for tweaking loads once they are close. Those things can certainly all be done without ever using such software, but the use of the programs can also make the process a little easier/simpler. I think most of those that use the software also appreciate having the "extras" that come along with the reloading software, such as various measurement outputs including the amount of bullet shank in the neck at a given seating depth, case fill ratio, predicted pressure, predicted effects of temperature on pressure/velocity, etc. I certainly do.

 

[RegionRat]

Keep in mind, OBT, or models that are only based on the type of steel and total length of the bbl, are only estimating the harmonics of one type of wave that is based solely on the speed of sound in the steel.

To shed light into the other structural waves and harmonics that have good and bad effects on us, the modeling would get much more complex in terms of the required inputs of dimensions, materials, damping, etc.

So all I am pointing out is that sometimes the reasons we get frustrated with over simplifications, even perfectly predicted and perfectly calibrated ones, is due to two concepts, one is that OBT attempts to estimate the frequency of the harmonic but has no way to predict the magnitude of that effect, and overall performance can be overcome by all the other harmonics and structural modes that it does not account for at all.

 

[JFrank]

The use of reloading programs such as QuickLoad or GRT can help expedite the reloading process largely because of the barrel time output and the availability of Chris Long's OBT node barrel time table. It does not preclude testing, but can facilitate finding the best charge weight region for initial testing. Whether one buys into Chris Long's explanation of the mechanism by which the OBT nodes exist (i.e. longitudinal shockwave theory) or not, too many people use these nodes to deny that they exist. An important observation about the OBT nodes is that they are relatively far apart in terms of velocity/charge weight/pressure. There are clearly additional "accuracy nodes" that lie in between the predicted OBT nodes. In my hands, there is generally a noticeable difference between the non-OBT, or "in-between" nodes and the OBT nodes, with the OBT nodes typically being more generous (wider) and possibly having even a little better precision.

It can certainly be argued that the use of barrel time in load development is no different/better than using ladder or OCW testing procedures that don't necessarily require any velocity measurements in order to interpret the results. That is why a number of shooters that employ reloading software such as QuickLoad or GRT use it primarily as a guide to choose a starting charge weight region for testing, and for tweaking loads once they are close. Those things can certainly all be done without ever using such software, but the use of the programs can also make the process a little easier/simpler. I think most of those that use the software also appreciate having the "extras" that come along with the reloading software, such as various measurement outputs including the amount of bullet shank in the neck at a given seating depth, case fill ratio, predicted pressure, predicted effects of temperature on pressure/velocity, etc. I certainly do.

Yes, nice for computer guy or perhaps a rainy day but we both know that a man of your experience can get a good barrel timing load put together with a piece of paper and glance at a re load manual. A node or window in between the OBT at least for myself would typically be smaller and more sensitive to breaking out. I would submit that a good tool like this has a place in today's shooting community as more and more people don't want to read a manual and create a hard copy / notebook they want to put it on screen and push save.
I like to get out doors and use the simple ways also pass on to those who ask what happens when the power goes out.

 

[Ned Ludd]

Yes, nice for computer guy or perhaps a rainy day but we both know that a man of your experience can get a good barrel timing load put together with a piece of paper and glance at a re load manual. A node or window in between the OBT at least for myself would typically be smaller and more sensitive to breaking out. I would submit that a good tool like this has a place in today's shooting community as more and more people don't want to read a manual and create a hard copy / notebook they want to put it on screen and push save.
I like to get out doors and use the simple ways also pass on to those who ask what happens when the power goes out.

As I noted, optimized loads don't require the use of reloading software. Shooters have been generating optimized loads a long time before reloading software was ever available, so its use is largely a personal choice. I choose to use it, but that doesn't mean it's right for everyone. In a larger context, I agree that it's important for those that enjoy the shooting sports never to forget that the simple enjoyment is what keeps us coming back. There are so many "bells and whistles" available for shooting-related sports today beyond just reloading software that it can be very easy to get caught up in obtaining whatever the latest "XYZ Mega-tiny Group Aid" or the mastering latest shooting technique and forget simply to have fun and enjoy it.

 

[Beiruty]

With GRT I have helped a benchrest shooter develop 5-shots in the same hole after 30rds. We could have done it in 19rds too.
He provide good measurents for the requested data