I would make a wild guess here that some bullets will not shoot good in sabots because they lack the center grip that rifling would engage. A perfect example is last weekend my friend and I shot one of his new Manlicker carbines with both old military rounds and some expensive handloads he bought. (spitzer boat tail bullets).

The handloaded rounds shot real bad in the old bore but the old military rounds shot fine. The only difference was the amount of area the old bullets engaged the rifling on the gun.

Think about it if the sabot can't hold the bullet straight the bullet ain't coming out of the barrel straight it need good bearing surface on the sabot.