====== Q: Given that all barrel and cartridge related data are correctly inserted (the usual length, combustion volume, bullet data) but you still see a measurable difference in estimated V0 and measured V0. What is the go-to parameter to start tweaking? ======

If the deviation is small, you just need to adjust the Initial Pressure (IP) until the measured and the simulated velocities are matching.

Alternatively the IP and the OBT-Tool in combination. The OBT-Tool can be used to just calibrate a powder model ignoring the OBT nodes thing.

====Safety Warning!====
**Be careful on Revolvers! With Revolvers you cannot do that because of the cylinder gap. It is very likely that it would give you a wrong / too low pressure estimation if you calibrate that way!**

The latest GRT version (as of Version 2020.1855) has the feature to include a leakage like the cylinder gap on revolvers to take that into account, only then you can calibrate the model against your measurements.

====Calibration with the OBT-Tool====

You can watch this video made by Cassie, which describes the process of calibration, you can ignore the computation to a specific node for just the calibration: 

{{youtube>XxDJVROdyxc|}}