This is the reason I built my RapMan :D
I've made a shell to go around a (heavily modified) Nerf Stampede to make it look like a 40k Bolter.
It's still fully functional, shoots darts and takes standard Nerf magazines (one day I'll do the Bolter magazine too)
Unfortunately it'll always be more a decorative piece, mostly due to it weighing 4kgs, making far too heavy to lug around all day during wars or HvZ :P
Not quite sure what's going on in the preview images for the .STL's, They printed out ok after a bit of rotating in Axon
(Obligatory Note: I don't own Nerf, or anything relating to GamesWorkshop/Warhammer, etc, I'm just a big fan of both)
The Stampede had to be minimised to make it fit, as shown in the picture comparing the lengths
For interest it's also running an aftermarket 9kg spring, and 2 banks (in parallel) of 4x Trustfire Lithium batteries for ~16v