I am wanting to put together a 5.56 PDW. I need an upper that will fit a milspec AR lower that I have already registered. What I am looking for is 7-8 inch barreled upper, but is designed and has a collapsible two rail stock like an MP5. It would need to be a gas piston system to eliminate the buffer tube. I know there are uppers that use impingement systems but have a shortened tube to allow a collapsible stock, but am hoping to find a receiver that will meet my above specs. I do realize there would need to be some significant mods to a standard upper to accommodate the retracted rails so maybe that's why I have had no luck so far finding what I am looking for. A complete upper would be nice, but am willing to build it if I can find just the upper receiver and stock. Any help or guidance on this would be appreciated.