You are correct, Andreas. The barrel turns in the hammer mill, or the barrel is stationary and the hammer mill rotates up the barrel. I believe there have been both types. Either way, this is how the rotational grooves are impressed on the outside of the hammerforged barrel. They do not come form the inside. Styre chose not to remove all of the marks at the breech end as they are rather sexy to some people. They merely smoothed, reduced and polished them.

Paul - extra stress on the barrel as the bullet goes up the barrel will increase pressure that is readable on strain gauges placed along the tube.

The transducer method of presssure recording actually shows the rise from zero pressure to peak over time and distance, as well as the pressure curve from chamber to muzzle. This is displayed on a graph. A single transducer should show comparative pressures for different bullets launched at the same velocity. This system has been used since the 70's by some outfits.