It is designed to work as a ambush weapon, the gun concealed by boxes and the awning which are both shed when it goes into action against raiders. 
The Master Engineer Luigi Giovanni, whose workshop is located deep in the Forest of Shadows had the delivery wagon of a Helblaster Volley Gun ambushed by raiders. He salvaged the remains of the Gun that had been pressed into service to defend the convoy atop the wagon before it exploded and inspired, for several months laboured to produce the first Helblaster Wagon. 
The Helblaster Wagon would later become a important escort vehicle for valuable wagon trains and the coaches of the Imperial Mail. 
Weapons and Equipment
The gun is mounted on a rotating platform which allows it to fire in any direction but as it does not have the special suspension of a War Wagon it is forced to stick to roads. Should it leave the road, the crew can do little more than hang on as it bumbps and grinds along. 
- 4th Edition: Helblaster Volley Gun, Four Human crew on Wagon pulled by two Warhorses.