I had an idea of using the CCS stock, a Double Diamond 9mm dedicated lower and a 5.5 inch barrel. I don't think I would ever build it though.
Double Diamond makes the coolest 9mm lower:
I just did a quick search, 4.75 inches collapsed, maximum extended length is 8.5 inches. I guess that's the price you pay for such a compact stock. It is pricey too.
IMO, the coolest pistol size carbine stock available on the market.
If I ever had the cash & incentive to build a pistol caliber carbine, I would for sure put the NEA CCS on it. You could make an AR15 the size of an MP5.