In the Horus Heresy, Blood Angels can replace Heavy Flamers with early pattern Assault Cannons. I needed a few for my rhinos, so I needed to kit bash some.
I started off with the left over assault cannons from the Storm Raven kit and a storm bolter from the armored container kit.
The barrel of the assault cannon gets attached to the front of the storm bolter assembly, the rear of the assault cannon gets repurposed as a magazine, and a small piece of sprue is attached to the top to hide the join and add some more bulk.
Here is the finished result. Still need to clean up mould lines and file down the edges, but you should get the idea.