Hmm there is something which i dinn knew. But a question how can a lateral spin affect the perpetrator rod ??? A normal common sense tells me that the entire round will spin on an axis which actually lies inside perpetrator rod . And yes u are true, riffled barrel dose need a complex shell design. Also u clarified one of my long standing doubts, about why dose even rifled barrel ammo needs to be fin assisted.
And about 120mm penetrators, yes those rounds cannot be used just off the shelf but i thought it was more to do with auto-loader carousel . ..
Because the rod is too thin at its cross section,it will tend to wobble at its lateral axis or get twisted along its longitudinal axis and might even break into pieces in mid air even before making an actual impact!!And as I said,lateral spin can only stabilize a projectile with a L ratio of 8-9:1,not more than that (I made a mistake on my above post when I had set the value at 6:1).
And no,the entire round doesn't spin........it should not spin!!The developers have to take special care so that the rod doesn't spin at all or the fins won't work.Just imagine the flight characteristics of an arrow,it's pretty much similar to a FSAPDS penetrator.
And lastly,no,it is not because of the carousels. Arjun tanks do not sport a carousel,so there is no question of compatibility issues.The sole reason is that the commercially available FSAPDS rounds are designed for 120mm smooth barrel guns and hence do not have those bearings fitted inside the sabots,which means they have no means of stopping those penetrators from spinning and breaking apart mid flight along its lenghth.You could still use the penetrators though,but the sabots has to be changed.