That is a violation of the law of conservation of momentum. True, there are forces acting sideways trying to bend the rails out, but there is a force trying to shorten the rails, as well. The gun cannot push the projectile forward without the projectile pushing back on something.
This is not like RPG where the projectile has a rocket driving it forward unless you have vaporized metal moving in the opposite direction as the projectile.
Well, obviously it doesn’t violate conservation of momentum, it’s just that the device is designed to transfer the momentum in such a way that you don’t see the classic backwards recoil.