I modded a regular SM500 for my friend a few months ago.
Basically, I used a hacksaw and cut off the barrel. First I cut up to the fake "flash hider."
I looked down the barrel to see that the air restrictor was even further down. Annoyed, I used the hacksaw and cut even further down the barrel. I kept going until I finally got rid of all the AR crap. I then put a 3/4" to 1/2" CPVC reducer straight onto what was left of the barrel. I epoxied that in place, then slapped a short 1/2" CPVC barrel in the reducer (a reducer is like a coupler except one side is one size and the other side is another size).
This is the only photo I have available, sorry for the lack of step-by-step instructions and such. Enjoy.
The gun in the top left is called R.A.W. Force. Does anyone know anything about this gun? Have one? Or know where I can get one?