Still don't understand OBT (Am I dense?)

[rustyinbend]

Just ordered QuickLoad to help me with load development. Trying to understand OBT and have read a lot about it ... but I'm not "getting it".

OK, there's a spreadsheet (and an online tool) where you enter your barrel length, and it spits out seven "Node" times in MS that you can enter into QuickLoad to help pick an optimal charge weight. I get that part.

What I don't get is why you'd pick one of those over another, or how you select which "Node" is the one you want to build to. Is the answer "Enter each Node value and build those seven loads."? That doesn't feel right for some reason, but it's all I can think of right now.

Can someone give me a plain language tip on (a) why OBT matters, and (b) how to pick the right "Node" to build loads to.

 

[rustyinbend]

I think I just answered my own question. Use the software to calculate OBT, then use the table of charge weights it gives you to map against the OBT from the tool (Excel or Online) to pick your target load, and then build up and down in increments to get the best loads to develop.

Right ???

 

[Holliday]

I think I just answered my own question. Use the software to calculate OBT, then use the table of charge weights it gives you to map against the OBT from the tool (Excel or Online) to pick your target load, and then build up and down in increments to get the best loads to develop.

Right ???

I have QL and find OBT to be interesting but not much else. Since I can find no reference to tolerance for OBT timing, I find it to be unfinished science.

QL itself is interesting but I find most of the powder burn rates significantly off. I bought it to save materials and find it has not done that and perhaps exasperated my use of incompatible powers.

 

[charnicus]

I think @OneShot63 can help.

 

[rustyinbend]

I have QL and find OBT to be interesting but not much else. Since I can find no reference to tolerance for OBT timing, I find it to be unfinished science.

QL itself is interesting but I find most of the powder burn rates significantly off. I bought it to save materials and find it has not done that and perhaps exasperated my use of incompatible powers.

Interesting ... I'm hoping for a better experience. I find myself "spit-balling" loads between Start and Max ... so I'm hoping to build some better structure and methodology around how I test powders, bullets, and loads. Every trigger pull in 300-WM, 300-PRC, and 338-LM costs a lot. If it gets me to proper ELR loads faster, then it pays for itself in about 50 saved cartridges. Time will tell ...

 

[Holliday]

Interesting ... I'm hoping for a better experience. I find myself "spit-balling" loads between Start and Max ... so I'm hoping to build some better structure and methodology around how I test powders, bullets, and loads. Every trigger pull in 300-WM, 300-PRC, and 338-LM costs a lot. If it gets me to proper ELR loads faster, then it pays for itself in about 50 saved cartridges. Time will tell ...

Well it’s a tool and like any tool, user experience is often dictated by the user. In my case that may well be me and my methods.

On the matter of OBT, I have never seen a science so detailed that did not at least discuss + or - tolerances. Perhaps this has been examined and I have just missed it.

 

[Bmghunter]

I have been trying OBT loads for a bit and found that the QL data and OBT data correlates nicely. The issue is, you need to fine tune QL to give you accurate and predictable results from your dope and chrono. Once I tuned QL, I found that my best groups matched OBT expected barrel times from QL predictions of barrel times. Of course, this is from a single gun and barrel experience, but accurate for multiple loads from it. However, I did not use QL to find that initial load, I ran QL and OBT in reverse, I chrono my loads (and fou d the best in the process) and then made QL values match my actual values by tuning the values. Once I got that, I viewed my best loads barrel time in QL and it matched the expected best times from OBT.
For my case, I have a 26 inch barrel and OBT says various timings should be best, lets just say for example (not real timings) .3ms .4ms and .5ms are best according to OBT. In QL, after tuning and gathering real data, I check the barrel times for each of my loads, ensuring QL gives the expected velocity matching my real data. Then I look at the barrel time and see what the values were. The best loads matched very well with OBT. For example, one good load would be very close to .4ms, and the loads slower or faster than this would be much farther away from .4ms than the best load.

 

[rustyinbend]

I have been trying OBT loads for a bit and found that the QL data and OBT data correlates nicely. The issue is, you need to fine tune QL to give you accurate and predictable results from your dope and chrono. Once I tuned QL, I found that my best groups matched OBT expected barrel times from QL predictions of barrel times. Of course, this is from a single gun and barrel experience, but accurate for multiple loads from it. However, I did not use QL to find that initial load, I ran QL and OBT in reverse, I chrono my loads (and fou d the best in the process) and then made QL values match my actual values by tuning the values. Once I got that, I viewed my best loads barrel time in QL and it matched the expected best times from OBT.
For my case, I have a 26 inch barrel and OBT says various timings should be best, lets just say for example (not real timings) .3ms .4ms and .5ms are best according to OBT. In QL, after tuning and gathering real data, I check the barrel times for each of my loads, ensuring QL gives the expected velocity matching my real data. Then I look at the barrel time and see what the values were. The best loads matched very well with OBT. For example, one good load would be very close to .4ms, and the loads slower or faster than this would be much farther away from .4ms than the best load.

Very interesting ... I have to ask ... If you're doing extensive testing to tune QL to match reality, then what good is QL? I'd think that without leveraging QL's "predictive" powers, it would not be especially valuable. Am I thinking about this incorrectly? It's a little like actually driving somewhere to determine how long it takes to get there, and then using that data to verify the estimate in your car's GPS.

 

[Bmghunter]

Don't want to get into the usefulness of QL, but I get that. That's a problem of QL, the default values are not necessarily accurate to your podwer and conditions.

 

[Holliday]

Don't want to get into the usefulness of QL, but I get that. That's a problem of QL, the default values are not necessarily accurate to your podwer and conditions.

I was actually hoping to be shown the error of my ways. I just am not seeing results that I had hoped for. It’s encouraging that you seem to have.

 

[Cascade Hemi]

I find single shot pressure ladders more effective than OBT spreadsheets, QL, or anything else. Load up 20 rounds in .2gr increments in the pressure region you want to be in and shoot them with a chronograph. Then run your three or five shot pressure ladder in your now known velocity range. Use QL diagnostically with your single shot ladder to back into the extrapolated load data for your three/five shot ladder. OBT is jivy at best.

 

[Hawk in WY]

I use QL in combination with OBT and don't find it all that complicated.

I start with the recognition that QL provides estimates which should only be relied on with a healthy measure of common sense and cross-checking.

I measure the H2O capacity of three cases fired in my chamber, not resized, and take the average. Do not attempt load development with virgin brass.

I measure the distance to the lands with the bullet I want to shoot and back COAL off .020 (.050 for Barnes bullets).

I enter what I can in QL: H2O capacity, barrel length, COAL, and select bullet and powder.

I pick a load that gets me decent velocity but well below maximum pressure.

I select the closest, lower OBT node for my barrel length from a table.

I increase the powder charge to decrease barrel time to that node. This inverse relationship is the closest to difficult this gets. I have learned to stay well away from maximum QL pressure loads. The QL pressure, velocity, and burn rate standard estimates are usually conservative I have found.

I also run the barrel time of entry analysis from QL's Propellant Table Setup to see if another powder might give more velocity for less pressure at less than a 105% fill. I also check the chamber pressure graph to see if Pmax is close to or equal to Z1 which I understand is a good indicator of a good powder.

If the OBT charge looks reasonable based on published data and pressure is well below max according to QL, I load five charges of for example 49.1, 49.4, 49.7, 50.0, and 50.3 (five loads for the five diamonds on my standard one-inch targets) if the OBT load is 50.0.

I shoot my five loads starting with the lowest watching for signs of pressure. I Chrono each shot.

I run the best load reducing COAL by 0.10 each time (0.25 for Barnes bullets) for another five loads if I don't see the accuracy I'm seeking.

This almost always gives me an acceptable group (<0.5 moa for hunting rifles, <0.25 for all others).

If not, I go back to QL with actual velocities and adjust the burn rate of my particular powder to match the QL estimated velocity to the actual, and start over with five more test loads based on the adjusted velocities.

This almost always gives me a good load in 25 to occasionally 50 rounds.

I have backtested QL against known good loads from before I had QL and they are usually close.

Works for me.

Good luck.

 

[rustyinbend]

I use QL in combination with OBT and don't find it all that complicated.

I start with the recognition that QL provides estimates which should only be relied on with a healthy measure of common sense and cross-checking.

I measure the H2O capacity of three cases fired in my chamber, not resized, and take the average. Do not attempt load development with virgin brass.

I measure the distance to the lands with the bullet I want to shoot and back COAL off .020 (.050 for Barnes bullets).

I enter what I can in QL: H2O capacity, barrel length, COAL, and select bullet and powder.

I pick a load that gets me decent velocity but well below maximum pressure.

I select the closest, lower OBT node for my barrel length from a table.

I increase the powder charge to decrease barrel time to that node. This inverse relationship is the closest to difficult this gets. I have learned to stay well away from maximum QL pressure loads. The QL pressure, velocity, and burn rate standard estimates are usually conservative I have found.

I also run the barrel time of entry analysis from QL's Propellant Table Setup to see if another powder might give more velocity for less pressure at less than a 105% fill. I also check the chamber pressure graph to see if Pmax is close to or equal to Z1 which I understand is a good indicator of a good powder.

If the OBT charge looks reasonable based on published data and pressure is well below max according to QL, I load five charges of for example 49.1, 49.4, 49.7, 50.0, and 50.3 (five loads for the five diamonds on my standard one-inch targets) if the OBT load is 50.0.

I shoot my five loads starting with the lowest watching for signs of pressure. I Chrono each shot.

I run the best load reducing COAL by 0.10 each time (0.25 for Barnes bullets) for another five loads if I don't see the accuracy I'm seeking.

This almost always gives me an acceptable group (<0.5 moa for hunting rifles, <0.25 for all others).

If not, I go back to QL with actual velocities and adjust the burn rate of my particular powder to match the QL estimated velocity to the actual, and start over with five more test loads based on the adjusted velocities.

This almost always gives me a good load in 25 to occasionally 50 rounds.

I have backtested QL against known good loads from before I had QL and they are usually close.

Works for me.

Good luck.

A lot to digest there. Saving this one for after my QL arrives, and I'll likely have more questions when I'm deeper into this.

 

[Hawk in WY]

Can someone give me a plain language tip on (a) why OBT matters, and (b) how to pick the right "Node" to build loads to.

I hope I answered your second question in my post above.

Here is the answer to your first question:

Optimal Barrel Time Concept

Let us know how it goes. We're all learning here.

Good luck.

 

[30cal user]

QL powder charges will be too high for over bore cartridges, but the Node is correct to the OBT, just something I have noticed, so if shooting wsm type cartridge, back down the charge weights 2-4 grains to match est velocity to start and then adjust the BA rate (burn rate) in QL to match your actual results

there are other nodes the bbl will like, QL tool just helps get you started and helps define max pressure so you don’t do anything stupid

 

[rustyinbend]

QL powder charges will be too high for over bore cartridges, but the Node is correct to the OBT, just something I have noticed, so if shooting wsm type cartridge, back down the charge weights 2-4 grains to match est velocity to start and then adjust the BA rate (burn rate) in QL to match your actual results

there are other nodes the bbl will like, QL tool just helps get you started and helps define max pressure so you don’t do anything stupid

What does "over bore" mean? I shoot 6.5-CM, 300-WM, 300-PRC, and 338-LM. Are any of them "over bore cartridges"?

 

[30cal user]

Yes, the WM, PRC and LM

 

[Cascade Hemi]

Or adjust the BA to match recorded velocity on a small fired sample with your powder and primer combination.

 

[Hawk in WY]

QL powder charges will be too high for over bore cartridges, but the Node is correct to the OBT, just something I have noticed, so if shooting wsm type cartridge, back down the charge weights 2-4 grains to match est velocity to start and then adjust the BA rate (burn rate) in QL to match your actual results

there are other nodes the bbl will like, QL tool just helps get you started and helps define max pressure so you don’t do anything stupid

I have seen that also.

My solution is to target the next level OBT barrel time down and work up to that until I have actual velocity data to adjust the burn rate. I also recommend cross-checking everything to published data preferably to data from the bullet or powder manufacturer.

I recommend not trusting QL's maximum pressure estimates until you have trued velocity to actual.

 

[Bmghunter]

What does "over bore" mean? I shoot 6.5-CM, 300-WM, 300-PRC, and 338-LM. Are any of them "over bore cartridges"?

It sounds like there will be quite a bit of QL research that you still need to do before effectively using it. QL can predict loads well, but you must enter very exact values, accompanied by real data to tune their default values in order to get accurate and SAFE loads.
This overbore concept directly affects the weighting factor value for quickload. There is a table I saw in another forum that lists the weighting factor values to use for each cartridge.
On the need to tune quickload, the default data they give you is based on the lot of powder or brass or bullets they used to gather the initial data, but it may not match your lot and therefore need to tune it.
Also tuning QL is not simply gather one chronograph speed or cartridge and change QL input to give you matching speeds. You need multiple loads and cartridges tested and make the QL input values work for ALL of those loads and cartridges.
After you have tuned QL to predict correctly based on your real data, only then could you confidently believe the predicted values to try them. SAFETY first.

 

[Hawk in WY]

It sounds like there will be quite a bit of QL research that you still need to do before effectively using it. QL can predict loads well, but you must enter very exact values, accompanied by real data to tune their default values in order to get accurate and SAFE loads.
This overbore concept directly affects the weighting factor value for quickload. There is a table I saw in another forum that lists the weighting factor values to use for each cartridge.
On the need to tune quickload, the default data they give you is based on the lot of powder or brass or bullets they used to gather the initial data, but it may not match your lot and therefore need to tune it.
Also tuning QL is not simply gather one chronograph speed or cartridge and change QL input to give you matching speeds. You need multiple loads and cartridges tested and make the QL input values work for ALL of those loads and cartridges.
After you have tuned QL to predict correctly based on your real data, only then could you confidently believe the predicted values to try them. SAFETY first.

Agreed.

If you can find that table of weighing factors by cartridge that would be very useful.

As I'm sure you know, the QuickLOAD manual is useless.

 

[Holliday]

There is a table I saw in another forum that lists the weighting factor values to use for each cartridge.

Sure would like to see that chart......

 

[Mark1954]

Is this the chart you’re referring to?

 

[Bmghunter]

Is this the chart you’re referring to?View attachment 7657236

thats the one im referring to yes.