As many video games attest to, sometimes firing a weapon doesn't have any effect whatsoever on the shooter's momentum, even in the cold, dead vacuum of space where there's NO OUTSIDE FORCE TO CORRECT THE FORCE DUE TO IMPULSE! Impossible, no? Well, in some cases, no, it's not quite impossible.
While many might immediately think of rocket launchers, which are self propelled, and therefore would have minimal effect on the momentum of the shooter, these are not classified as recoil-less weaponry in the traditional sense of the term. Recoil-less weapons, specifically rifles, fire modified artillery shells which still behave as a simple projectile after leaving the weapon. How does this work, then? While a normal rifle would cause the gasses inside the bullet to expand in a closed chamber, propelling the bullet forwards at high speeds out of the barrel, recoil-less rifles allow for some of the gasses to escape out the rear of the rifle, compensating for most of the recoil which would normally result from launching a projectile.
This actually allows for two different side effects. First, this allows for the removal of heavy recoil mechanisms and the reduction of weight of the barrel. Second, it reduces the velocity of the launched projectile. In order to compensate for this, most recoil-less rifles launch a much heavier projectile a shorter range, in order to take advantage of the relatively light weight of the rest of the weapon.