Finally got around to modeling the Harkonnen rifle today.
I only have one good reference images of the prop and some stills from the movie to work from.
Still need to tweak some of the details/proportions.
Examining the movie stills, it looks like the prop was hard to hold as the actors resort to holding onto the thick "wires" above the stock and the handle at the side (not visible in this render). There seems to be some confusion among the actors about which way up to hold the rifle - When Piter arrives and walks down the stairs all the guards are holding the guns with the wires facing down, while the two guards in the Baron's room that bring in the boy are holding the rifles wire size up.
Also IMHO, the magazine is waaaaay to small for a weapon with five barrels!
Need to do the Harkonnen pistol next.