Rotational acceleration can be calculated but like MD and others wrote, it’s insignificant compared to other factors such as linear acceleration, deformation of the jacket due to rifling/friction.
The main thing I don’t get for sure is the secondary explosion which is uncommon and I have not read a credible explanation or maybe I just forgot. But here’s my guess: the friction is great enough to reverse the acceleration and pressure builds/spikes enough to become an explosion. I suspect this happens mainly with a powder charge that is in away from the primer and the powder doesn’t sufficiently ignite until a short time after initial ignition. From what I’ve read, it seems to occur mainly with partially full case of slow burning powder if I recall correctly.
My apologies to others who explained this phenomena better than this.
The guy you would have wanted to discuss this was Jim Ristow, owner of Recreational Software and designer of the Pressure Trace. He was pretty proud that his device documented several instances of Secondary Detonations. He'd talk your ear off about it if you gave him the chance. I wasn't too interested in the phenomenon back then, so I can't tell you what his theory was. Like in so many other things, I wish I had paid more attention.
