+1 on the avatar...
When the .223 was first developed for military use the idea was to take a small bullet at high velocity and over stabilize it so that when it hit something the bullet would begin to yaw and tumble. that's how in the early days of the M-16 people would say that the bullet tumbles in flight. It would, after a fashion but only after it hit something.
Velocity aside at 35 yards that could still have been what happened with your round. Hard to say. The real question is, how accurate is the round?