The 9mm bullets don't tumble at that range, they are still going straight even though they drop below the speed of sound at less than 100yds. If simply dropping below the speed of sound caused tumbling than the effective range of the 9mm parabellum would be 80-90 yds. The difference in the effect of passing the sound barrier between these short bullets and the longer rifle bullets is the center of gravity's location in relation to the aerodynamic stability of the bullet. With the short pistol bullets the CG is pretty close to the aerodynamic center and therefore the bullet does not lose much in the way of stability as the shockwave moves forward due to the slowing of the bullet. In the long rifle bullets the CG is frequently quite a way back on the bullet and the CG always wants to be in front. The only thing keeping these bullets going straight is the spin and aerodynamics. As the bullet slows down the shock wave moves forward and that destabilizes the bullet as it moves in front of the CG. This doesn't always lead to tumbling but can cause wobble or nutation as its called. That is what disrupts the accuracy, not tumbling. Sorry for hijacking the thread with ballistics but it is pertinent.....
Frank
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