Here's how I would do it:
(I'll just put Brother Grytt here so you can see what I'm referring to)
I'd take the
GK Psilencer as the base gun, cut off the tri-barrel and add a length of Plasticard tubing for the main barrel. Then I'd add a slightly larger diameter piece of tubing over the very end for the muzzle shroud (The black bit at the very tip of Grytt's cannon). Next, cut 4 smaller diameter plasticard rods and attach them around te barrel as the recoil dampeners. Finally, where the cables go into the Psilencer, cut them back a bit and fit an ammo box from the Devastator kit.
If you'd like, you could detail it a bit further by making the recoil dampeners out of plasticard tubes and fitting smaller plasticard rods inside to make it stepped - like a real piston. Also, you could make up a belt of frag rounds from your tubing and position them as though they are coming out of the box and into the Psilencer (You could also slide rods into really thin sections of tube to mimic the links that hold the rounds together)
So you could do this with:
A pack of assorted diameter Plasticard tubes and rods.
A Grey Knight Psilencer.
A Devastator Ammo Box.
Hope that helps.