Easy with loads of money to burn. take the asset . upsacle it in CAD and send it to shapeways to get 3D printed.
A plasma pistol is about 700 bucks last I looked a year ago..
Now for more practical means.
I use a combo of NERF blasters and I have my own printer so I just bodykit it in to a halo prop.
You can also use EVA foam and use 1/4 in wood and carve it a tad too.
Or pep it from the pep archive. . however besides a few fan made files . To my understanding there is NO Halo 5 weapons out yet.
you might have to settle for H4 stuff for now or freehand it from screen caps from 5
The H4 and the H5 shotguns aren't that different from each other (I think the only difference is the flashlight on the side). I would say though, like Peter said, using a Nerf gun for a base then mod it using foam, PVC or wood. I wouldn't buy a brand new NERF gun though, check out a pawnshop or a goodwill they could have some for a good price. Also, just look around on youtube and/or forums here some tutorials I'm sure at least one exists.
Personally I'd just build one from eva foam from scratch. It's easy to work with, convention friendly, and can look great if you put some effort into it! Just be sure to run a wood dowel through the core to give it some rigidity!