M193 was NOT designed to tumble - all pointed and many semi-pointed bullets yaw in soft tissue because the bullet's center of gravity is located toward the base. The spin stabilization used to stabilize bullet flight in air is insufficient to keep the bullet penetrating point forward when it penetrates flesh. Soft tissues are 700 times more dense than air. When the bullet penetrates soft tissues it seeks to achieve a state of stability. It does this by yawing 180-degrees to travel base forward. The bullet does not tumble end over end. Both M193 and M855 yaw. If penetration velocity is high enough they both will fracture at the cannelure as they yaw through 90 degrees. The cannelure is a weak point in the jacket and the stress placed on the bullet as it penetrates sideways, depending on velocity, will flatten the bullet causing to to extrude lead out the open base, and fracture at the cannelure and fragment. The fragments pepper surrounding tissues with small holes that are then torn open by the subsequent temporary cavity.
The issues with mild wounding effects caused by M855 are due to the fact that it is yaw sensitive. Some rifles do not stabilize the bullet as well as others and this causes the bullet to wobble in flight. This effect is called fleet yaw. In addition, because M855 is a compound bullet, with a steel tip and lead slug in its core, there are manufacturing variations that cause individual bullets and lots of bullets to be less stable than others, and this also causes the bullets to wobble in flight. Fleet yaw and bullet variation affect the bullet's angle of attack. A bullet that penetrates point forward will penetrate deeper before it yaws whereas a bullet that hits at a slight angle because it's wobbling will yaw at a shallower depth. Finally M855 was designed to be fired from the 20" barrel of the M249 SAW and M16A1. The shorter 14.5" barrel of the M4 carbine decreased velocity substantially which substantially decreased its wounding effects.