According to the law of Conservation of Momentum, there is no way to increase the momentum of a system, except by momentum transfer from interactions with the external. If I fire a rifle while sitting on a go kart, the go kart is going to go backwards but the bullet goes forwards, conserving the momentum.
Now lets say I construct a long 1 inch thick steel box (a few meters long), and I position the gun's butt against the back of it, and fire the gun electronically. Would we not get the box flying backwards still (at least until the bullet gets lodged in the front of the box? Even if the bullet burying in the metal at the end of the box causes another force in the box at the opposite direction of the initial kick, haven't we momentarily broken the conservation of momentum?