division is (uncountable) the act or process of dividing anything while separation is the act of disuniting two or more things, or the condition of being separated.
Separation is the propagation method in which these natural structures are simply removed from the parent plant and planted to grow on their own. When one of these structures is cut into sections that will then grow into a new plant the method is referred to as division.
The total momentum is unchanged according to the law of conservation of momentum. When the gun is fired, the bullet gains a high velocity forward (positive velocity), and that velocity multiplied by its mass is the momentum the bullet gains. Therefore, the gun must gain a momentum backwards to cancel out that momentum forward, so the gun recoils back with a negative velocity.