The Jackhammer, despite its name, is an easy to control shotgun even when fired on full automatic. The popular bullpup design, which places the magazine behind the trigger, makes the weapon well balanced and easy to control.
In real life, the Jackhammer was a prototype design that failed to reach full-fledged production due to its manufacturer's bankruptcy. Very few functional prototypes (according to some reports, as few as two) were ever made. The game stays true to the weapon's reputation of controllability, even with automatic fire.