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.

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.

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: