A Helstorm Rocket Battery has the following profile and rules:

A Helstorm Rocket Battery fires in the same manner as an ordinary Stone Thrower, except that you scatter 3 templates from its target position rather than 1. When scattering any templates after the first, only the scatter dice is rolled; the result from the artillery dice first rolled applies to all shots fired that turn. In addition, all shots made with a Helstorm counts as being fired indirectly. If a Misfire is rolled, roll immediately on the Black Powder Misfire Chart.