[Kamog]

Regardless of how the object is propelled, whether it be with a string of magnets, or whatever, a force was applied to it in order to cause the acceleration. Whenever there is a force acting on an object, there exists a reaction force of equal magnitude in the opposite direction.

Suppose that you have a strong electromagnet in the launcher, which you power on. An iron projectile is attracted towards the magnet, and starts accelerating in that direction. The force of attraction does not occur in just the single direction - from the magnet to the projectile; the force is equally strong in the opposite direction - from the projectile to the magnet. The reason that the magnet doesn't seem to move much is because it has a much larger mass (and attached to a support that is also a large mass), and therefore the same magnitude of force causes only a minor acceleration backwards. The projectile, having a small mass, is accelerated faster with the same magnitude of force.

You can't pull an object towards you without the object also pulling you towards it. You can't push an object without that object pushing back on you.