The Vihta Vuori model works quite well at predicting pressure/velocity figures up to a point. After that, expected results will differ widely from actual. Unfortunately, there is not really any way of knowing where that point actually is, without measuring both pressure and velocity. Usually the model is fine for predictions of pressure/velocity at least up to what is generally considered to be +P+ pressures. I use the model, but I also measure.GBullet said:Internal ballistics as it relates to pressure issues is a lot less predictable than most engineers would have you believe. Loads commercially sold are carefully developed by ammunition makers.