Let's define what you mean by "tumble". The 5.56 bullet does NOT tumble in flight. If it did you would not be able to hit any thing and bullets would simply zip around every which way. Now the pointy shape of a bullet is unstable. The heavy base end wants to switch places with the lighter pointy end. To stabilize the bullet in flight the rifle barrel has twists inside to cause the bullet to spin. The first AR15s had a 1 in 14 twist rate this rate of twist worked in temperate climates but was insufficient for cold weather. In the cold dense air the bullet did tumble in flight. Thus it was changed to 1 in 12 for all weather operation. The M16A2 further increased the twist rate to 1 in 7 for 62gr M856 tracer bullets with light pyrotechnic compound making them longer than all metal M855 62gr FMJ, with M855 already longer than 55gr M193. Keep that in mind: spin and density - insufficient spin in greater density causes the bullet to yaw as the heavier base tries to switch places with the lighter pointy end.
Thus rifle bullets tumble after entering flesh because they are not spinning fast enough to remain stable in matter denser than air. With an aft center of gravity from a base wider than their tip, their inherently stable traveling orientation is base first, which they rotate into when they have insufficient spin. Therefore all FMJ rifle bullets will exhibit some degree of yaw when they transit media. A case in point would be the "magic bullet" that killed Kennedy. This slow-moving round-nosed 6.5mm bullet exited his body and struck Governor Connoly (who was riding on the jump seat in front of Kennedy.)
The hole in Connoly's jacket is clearly a keyhole -- indicating about 90 degrees of yaw.
Now add to that the construction of the bullet. M16 bullets had thin jackets with heavy cannelures, so when they began to yaw on impact, they would sometimes break apart at the cannelure. Depending on conditions and location of the hit, this might result in a spray of jacket and core metal through the heart-lung area, or might simply be a gory wound in muscle tissue.
Any projectile that is traveling approximately 2500 fps or faster can cause hydrostatic shock when it hits soft, moist tissue because the water inside the tissue is non-compressible, and as the shock wave travels through tissue it bursts the cells, causing massive wound channels.
The Soviet 5.45 X 39 uses a bullet with an empty nose and a flat-pointed core (supposedly intended as a penetrator for armor). On impact, the core comes forward in the jacket, filling the nose, and causing the bullet to yaw rapidly -- producing some really sickening wounds in any tissue.
