dstoenner
Silver $$ Contributor
First) A true story. My friend got his 243 rebarreled (by me) and he took a couple of loads to the range. Unfortunately he hit high pressure with one but not terrible. He gave me his velocity and powder level and I plugged them into GRT. I then did an OBT and yes GRT said there were 103%. I then ran an OBT that would be lower pressure and gave that to my friend. He loaded some at the projected target value and some at .2 gn higher. When he shot them they didn't group and when he gave me the velocities, they were 200 fps slower than predicted.
The light bulb came on so I inputted the new velocities and did an OBT of the new values and the powder parameters Ba and K were lower values and predicted the new powder points perfectly and had a Pmax -15% node at 2 gn 1 gn more.
SO here is the caution. My conclusion is that OBT is a piecewise linear approximation to the actual curves of BA and K which are not linear. And where you run OBT WRT the center of the curve will give you radically different values as you try to extend it lower or higher.
My recommendation for a strategy here to find a good first order solution is to take the base value of powder from the table with att of you inputs on your gun and case volume and find a node somewhere in the 15 to 25% band. Now load 5 rounds at that value and 5 rounds at 1 gn more or less to establish to points on the curves and run OBT a second time to get a better understanding where your nodes might be. Then test the node with a ladder that has that node somewhere in the steps. You should get a good node to then start with.
Second issue. I have also figured out that GRT has some filter on when it will modify the powder parameters curve fit and when it will just leave them alone. The only way to force it to do a real curve fit to you parameters is to go back and reselect the powder from the internal tables and then do the OBT. I back annotate all my runs If you do not back annotate the Ba and K values then you don't have to worry it will ALWAYs curve fit for that run of OBT.
Hope this helps
David
The light bulb came on so I inputted the new velocities and did an OBT of the new values and the powder parameters Ba and K were lower values and predicted the new powder points perfectly and had a Pmax -15% node at 2 gn 1 gn more.
SO here is the caution. My conclusion is that OBT is a piecewise linear approximation to the actual curves of BA and K which are not linear. And where you run OBT WRT the center of the curve will give you radically different values as you try to extend it lower or higher.
My recommendation for a strategy here to find a good first order solution is to take the base value of powder from the table with att of you inputs on your gun and case volume and find a node somewhere in the 15 to 25% band. Now load 5 rounds at that value and 5 rounds at 1 gn more or less to establish to points on the curves and run OBT a second time to get a better understanding where your nodes might be. Then test the node with a ladder that has that node somewhere in the steps. You should get a good node to then start with.
Second issue. I have also figured out that GRT has some filter on when it will modify the powder parameters curve fit and when it will just leave them alone. The only way to force it to do a real curve fit to you parameters is to go back and reselect the powder from the internal tables and then do the OBT. I back annotate all my runs If you do not back annotate the Ba and K values then you don't have to worry it will ALWAYs curve fit for that run of OBT.
Hope this helps
David
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