This isn't a straight up mod guide, but more of what I'd call an "internals tour." That said, this should give you an idea on how to do this if you are interested. The reason I made this thing is cause I got this Roughcut in a free bin at a nerf war, and it was stock, but was lacking air restrictors. So I singled it. Or would it be doubled...?
Step 1: The Basics
Here, you can see that this is basically just a Roughcut without intelligent ar's. I've applied a paint job, and put in two 1/2 inch pvc couplers, so I can attach a barrel.
Step 2: The Barrels
What I did here is take off all the junk on the front, and dremel put some holes where the couplers should go. On the back end of the couplers, there is some 1/2 inch pvc wrapped in electrical tape, but you can't see that because it's hidden by adhesives I used. I used pvc cement to attach the couplers first, the added some epoxy to act as a sealant and to make it look a little better.
Step 3: Plunger Rods
Not much going on here, although I did scratch up the face of the plunger tube with a screwdriver so that when I put in hot glue it would stick better. The hot glue is there simply for a little dead space removal.
Step 4: Shell Modifications
Here, as you can see in comparison to another Roughcut, I have cut off the front parts that would have been in the way of the barrels. For this, I just made some crude sharpie measurements, and cut those off with a bandsaw, then used a belt sander to smooth off the edges, and finally de-burred the edges with a razor blade. But seriously, be careful with those things! I've got some stitches as a result of being careless with one!
Step 5: Conclusion
So, that's a pretty simple explanation, and it may have not been the most in depth, but it should give you some guidelines if you want to try this. Thanks for reading!