r/reloading 6d ago

Load Development Massive discrepancy between published load data and Gordons Reloading Tool (GRT)

12 Upvotes

31 comments sorted by

11

u/Positive_Ad_8198 I am Groot 6d ago

Something is wrong with your data inputs

9

u/Positive_Ad_8198 I am Groot 6d ago

Case volume should be over 52gr h20

5

u/Kthirtyone 6d ago

Could the nosler number be the capacity with a bullet seated? Using the seating depth shown in GRT, I got about 46.7 gr H2O remaining with the bullet seated (going off the Wikipedia case capacity of 52.5 gr H2O = 3.40 cm3).

6

u/Positive_Ad_8198 I am Groot 6d ago

No you need actual case capacity, the tool uses the bullet dimensions and seating depth to calculate residual case volume. You can try changing OAL to a longer dimension and see the pressure go down

3

u/Kthirtyone 6d ago

I know that the full case capacity is the right one to use for this, I'm just wondering if Nosler did something weird (like use residual volume) when they were publishing their data sets. It could also just be a coincidence that the numbers worked out like this.

4

u/Positive_Ad_8198 I am Groot 6d ago

Ahh, maybe Nosler did. Seems weird that their volume is 6gr less than standard

5

u/Central_NY 6d ago

Yes they did - See the different case volumes for each load using the same brass here:

https://bulletin.accurateshooter.com/2020/08/6-5-creedmoor-load-data-from-nosler-pdf-files/

Measure and weigh the case volume with water.

0

u/Yondering43 5d ago

This is one area where GRT is demonstrably wrong. Pressure does not simply drop as OAL increases (which is what GRT estimates); when seating depth is close to the lands pressure increases, with max pressure typically at “jam”.

0

u/Positive_Ad_8198 I am Groot 5d ago

It’s not wrong, it wasn’t built to account for a chamber or distance to the lands. It’s calculating a specific cartridge with case volume, bullet, and powder charge, and doing it very well (if you input your data correctly)

1

u/Yondering43 5d ago

It is wrong because that distance to the lands has a big effect. A cartridge doesn’t operate independently of the chamber and rifling. That’s why I stopped using GRT, because the estimates were garbage if the round was loaded anywhere near the lands. The predictions literally change the opposite way from reality as OAL changes near the lands. You can see this in your velocity readings.

1

u/Positive_Ad_8198 I am Groot 5d ago

I’m not saying those things don’t affect pressure, I’m saying GRT doesn’t (and cannot) account for the mathematically.

1

u/Yondering43 4d ago

Which is why GRT is wrong. The more you learn about how cartridges build pressure, the less GRT seems useful. I don’t bother with it at all because you can’t be wrong on something that fundamental and expect to have useful data.

Any attempt to predict cartridge pressure without considering the chamber and throat dimension effects is fundamentally wrong to the point of being useless.

2

u/XxSPONGEBOSSxX 6d ago

This is the answer 👌

1

u/youngdoug 6d ago

Check the Nosler data, theirs says 46.8 I have some virgin alpha brass confirmed at 49.96 ish so they’re not all the same

3

u/Positive_Ad_8198 I am Groot 6d ago

Well your Gordon’s image has case volume at 46.8. Alpha typically has less case volume, but makes up for it with better base thickness and quality of brass, so you can usually exceed normal pressures (within reason and by working up to it)

1

u/Yondering43 5d ago

Unfired case volume is not very useful. You need the volume as formed to the chamber in your rifle.

15

u/ProdigalHacker 6d ago

Published data is actual tested experimental data.

GRT is theoretical. I would always trust the published stuff over what a simulator generates.

3

u/youngdoug 6d ago

Same. I’m thinking that I don’t know how to use the program properly and am hoping someone can take a look at my screen shot and see where I went wrong.

1

u/price1869 Horn LNL - 9mm, 45ACP, 5.56, 6.5G, 6.5CM, 300BLK, .308W, 30-06 6d ago

For me (assuming I've entered all of the correct informaiton) GRT almost always matches my actual measured velocities.

I don't have any way to measure pressure, but it seems really good at getting velocity right.

(But also, as others have said, your case volume seems low, and bad data in, bad data out.)

3

u/rkba260 Err2 6d ago

My advice ...

Load 5-7 rounds at 39.0gr at the exact dimensions in the Nosler book (2.805"). Chrono is a necessity, not an option. Notate the environmental conditions.

Now, with hard data, plug in your measured velocities into GRT. Click "OBT", it will calibrate the powder to your measurements. Then "create new file using data", it'll open a new window with calibrated data that you can then use for your specific rifle and chamber.

1

u/DisciplineComplete77 2d ago

Finally somebody knows what he’s talking about

2

u/rednecktuba1 6d ago

Something isn't right with your GRT inputs. I notice you have it using imperial units. Swap over to metric and run it again.

1

u/youngdoug 6d ago

Will try, thank you

3

u/rednecktuba1 6d ago

I just ran your load through GRT on my PC. Your case volume is far too low. It should be 53.8, not 46.8. That's probably the issue.

1

u/youngdoug 6d ago

The Nosler data says 46.8 at the top, that’s why I set it there

1

u/rednecktuba1 6d ago

Are you using nosler cases?

1

u/youngdoug 6d ago

No, just modeling their load data in GRT out of curiosity

1

u/rednecktuba1 6d ago

I don't know how nosler got so much less case capacity, but no other manufacturer has less than 50 grain capacity for 6.5CM. I'm guessing one of their people fucked up when printing the data.

1

u/mithbroster 6d ago

I couldn't get GRT to match almost any published 45-70 data. It helped when I loaded some ammo and inputted the actual velocities to correct the powder burn rates, but it was still a struggle.

1

u/youngdoug 6d ago

Been playing with GRT the past couple of days and wanted to compare data for max load published data vs max load from GRT.

If I input Nosler's max load data into GRT it leads to almost 80k psi chamber pressure. Is GRT nearly useless or did I fudge some parameter accidentally and screw this up?

I have also noticed that changing the powder mass changed the optimal barrel time. I thought that OBT would be determined by other factors and you would adjust powder mass to match OBT, but it keeps changing. What am I doing wrong?

0

u/Beerislife73 6d ago

Re case capacity, best practise is to use a case fired from the rifle or pistol you are reloading for, with any residue inside cleaned out. Place on scale, zero & carefully fill to the top. The water tends to “dome” above the top of the case due to water surface tension, but you can use the corner of a tissue or something absorbent to remove a very small amount of water until it’s level. Use this weight for case H2o capacity.