Looks like a gun for a giant robot.

The A'rstear (アールスティア Ārustia) is a Semi-Autonomous Companion Gunner Ship used by the Triggerheart Exelica in the game of the same name. It's her companion ship and primary weapon, it floats on her right side

This ship can destroy targets using light spheres, and their flight paths can be controlled after firing.

After landing on Earth, Exelica adopted it as her flagship.
Referreing to it as "ship" based on the concept of forming a small fleet with Exelica along with the D'rfend.

A'rstear 02

Another view of the A'rstear

Unlike conventional weapons that are held in one's hands and fired with a trigger, A'rstear is controlled entirely by Exelica's mind.

Firing and acceleration orders are supplied by Exelica.
Only a limited supply of bombs (six to be exact) are available for attacking a wide area.