bullets are haunted by the same problem as muzzle crowns. if a hip of the tail of a is not concentric than gas will excape on oneside before the other forcing the tail to wag. fbs are easy to square while bts are hard to square.
another problem with bts, at the same point as the wag, is that instead of the gasses being blowen away 90 degrees to the bullet like with a fb they are allowed to flow around the bullet creating turbulance infront of the bullet.
fbs have their own problems like excessive drag and pressure build up behind the bullet in ss flight