This surprising handmade mobile suit is made by forcibly attaching a heavily damaged Zaku to a Rick Dom.
This machine was used in clan battles and is said to have been developed by the military during the War of Independence (citation needed), and its specifications are quite different from the regular Dom Cannon.
The Giant Buzz is simply attached to the Zaku's shoulder.
The magazine position has been changed to ensure a good firing angle.
It is also equipped with a shield that appears to have been plagiarized from somewhere.

HG Rick Dom Cannon Custom (GQ) | フィリョース's Gunpla works