The A very large old-timy rifle on the back of some old-timy truck is a vehicle in Just Cause Unity.
This is a Mario Frigo special, as he literally found some unidentified truck in a barn somewhere in Medici, and then got access to some wood and metal pipes. Mario then assembled this thing, and didn't bother to name it for some reason, so everyone just uses how Mario explained it: "A very large old-timy rifle on the back of some old-timy truck.". The truck has only seen use as an artillery unit, as it is rather big.
The vehicle is very slow, and likes to roll if it is turned to sharply. The gun is around twice as powerful as an M488, but it is harder to aim.
- The gun featured appears to be some kind of oversized-repeater style rifle on top of a old truck, something that would be highly impractical in the real world.