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.
