The problem is indeed with the rifling, but not because of sabot separation issues, it's because of insufficient spin imparted on APDS round to ensure its stability.
Oh, that meets it more exactly.
The Kuerassier gun, on the other hand, requires *specially modified* APFSDS rounds, with reduced charge compared to normal French rounds. It cannot fire standard 105mm APFSDS rounds as far as I understand.
The APFSDS for M-57 of the Kuerassier has shorter cartridge case. The return way had to be kept smaller. The APFSDS projectile is however as with the F1, if I see it correct. The muzzle brake had made problems when shooting with APFSDS, therefore it was changed (two chamber to one chamber muzzle brake).