I could see using the front piece for the base of a carbine of some type. Add some PVC for longer barrels, add greebles and whatnot and it could become a respectable gun. The main gun, follow Cando's advice: chop, bondo, add PVC and you could be well on your way to something. It could either become a heavy blaster rifle or sniper rifle depending on what you add and subtract from it. Look at what others have done and use bits and pieces of anything you see and like out there. Also, don't be afraid to cut peices off to completely alter the look of the weapon. This way not only will it conforms to the new Nerf CRLs it will be uniquely yours.
For example I absolutely butchered my longshot front half and used parts of it for my carbine which I combined with a Nerf Recon and a Droid blaster. Then I atttacked the the main gun with my dremel and used pieces towards a DC-15a clone trooper style long rifle that I'm working on. The extra parts from it I donated to my other michigan vode for their own projects.
Good luck on your build.