I think Newton’s third law of something pushing back with the same force that it pushes forward could be a problem as well.
If they are discussing a railgun rifle maybe they have a prototype or close to one that we are not aware of.
Science Fiction always becomes science fact.
With railguns, most of the recoil is actually perpendicular to the direction of the projectile, trying to force the rails apart in both directions. If the rails are bound to each other, the forces net against each other, and basically there is no recoil.