Take a NERF N-Strike Barricade RV-10 and then re-work it completely. Etsy shop operator Faustus70 stepped up to the challenge, adding cut planks of wood from an old chair, a scope, access pannels so you can get to any jams and building, in their words, a big daddy of a steampunk NERF gun.
“You won’t find an other like this any place,” notes Faustus70. I suspect they may be right.
Anyone want to tackle the question; “Is this really steampunk?”