The firearms shown in Upgrade embedded in the arms of several antagonists are incredibly unrealistic. The weapons appear to have no recoil in the movie, which is not realistic, and appear to also be quieter than most modern firearms and fire bullets with enough force to pierce metal walls [1]. The forces involved would cause serious injury for the weapon's owner, and performing maintenance would be virtually impossible. Recoil-less weapons do exist but they use other methods to vent the force produced from firing such as allowing the explosive charge to escape out a hole in the back of the gun and this is not seen in the movie.
Daniel Stusalitus