GRT Question

[Martin52]

After inputting velocity data and comparing the average with the predicted velocity I find there is usually a variance, and particularly so when the actual velocity data relates to small rifle primers. This can mean quite large changes in initial pressure to align actual with predicted velocity and I don't feel is the right way to go.

My question is how do you align actual and predicted for OBT ?

Do you

a) adjust initial pressure to align actual with predicted velocity (large changes may be needed for srp's) or,
b) do you run OBT, cut the OBT values for K and Ba and paste these to replace the K and Ba values on the main page - then make very minor adjustments to IP if needed

I have watched Cassie Nienaber's video on GRT and he seems to adjust both IP and K/Ba.

Having experimented with a) and b) above, I have come to the conclusion b) is the correct procedure.

 

[dstoenner]

Answer B above is correct. Obt is preforming a curve fit and adjusting those 2 values to your entire rifle and load as a system.

Check this out

How to get a load in 50 bullets with Gordon's Reloading Tool (GRT)

There have been a few threads on here about GRT. I have been using it for over 2 years now. As an electrical engineer, I am used to simulating systems to see the response. GRT is just exactly that but the "trick" is how to coax it into giving you a good answer. That took me some time to...

forum.accurateshooter.com

David

 

[SeabeeKen]

Click on OBT. input your measured velocity in the appropriate section and click compute OBT. Then click create file copy with the new powder values. A new tab with the calibrate powder model will appear. If you like, you can save the new powder model in the user section for use later. I have a number of calibrated powder models for my rifles.
All this is in the directions for GRT which can be downloaded from their website.
I dont mess with start pressure unless changing bullets. I calibrate the powder model to whatever primer I'm using at the time. Hope this helps

 

[Martin52]

Thanks both for your replies and the referral to the Jan 2024 thread. I have looked back over my past GRT models and find the missing link is not creating an OBT file - I previously just used the OBT result based on actual fps data and actual powder charge. I am going to re-run GRT with new data when I can get out and shoot some more, soon. Interestingly though, my early work with GRT gave a node for the 200.20X in .308 using 43 gr Varget, ~ 2605 fps, which whilst on the slow side, produced very tight groups. COAL was 3.035 in a 31.7 inch 10T barrel.

 

[SeabeeKen]

Thanks both for your replies and the referral to the Jan 2024 thread. I have looked back over my past GRT models and find the missing link is not creating an OBT file - I previously just used the OBT result based on actual fps data and actual powder charge. I am going to re-run GRT with new data when I can get out and shoot some more, soon. Interestingly though, my early work with GRT gave a node for the 200.20X in .308 using 43 gr Varget, ~ 2605 fps, which whilst on the slow side, produced very tight groups. COAL was 3.035 in a 31.7 inch 10T barrel.

Speed doesnt necessarily go with accuracy.The OBT can calculate the next higher accuracy node and powder charge. Gordon said he felt that the OBT was only 40% accurate but my massive amount of testing in many rifles find it is more like 70% and usually within a few tenths even when its off. good luck, enjoy

 

[jelenko]

usually within a few tenths even when its off

Same here.