Etsy seller Steampunk101's GOLIATHON is a truly shitkicking steampunk Nerfgun mod, though that craftspersonship doesn't come cheap: $350!
What happens when you weaponize all the horsepower of a full-size steam locomotive? You get the Goliathon. One shot can level a building, down an airship, or turn a man inside out.
This is a modified Nerf Vulcan. It has many real metal elements added to it, including 4 gauges, copper piping, brass embellishments, and a solid metal valve wheel. It is roughly 2 1/2 feet long. It is a prop only and not intended to fire anything.