The intended behavior was that exactly one of the shells (the faster one) would run the collision code and react accordingly by moving backwards until the shells no longer collide and set the speeds of both shells to go to opposite directions. What can happen is that the shell teleports to the other side of the collision partner shell instead and the two shells somehow collide repeatedly each frame until one of them is destroyed.
The intended behavior was that exactly one of the shells (the faster one) would run the collision code and react accordingly by moving backwards until the shells no longer collide and set the speeds of both shells to go to opposite directions. What can happen is that the shell teleports to the other side of the collision partner shell instead and the two shells somehow collide repeatedly each frame until one of them is destroyed.