XCR pistol? Just remove the stock, and voila!
As for barrel length, under the current gas setup, you will only be restricted by the gas valve and block. I haven't taken a caliper measurement yet, but I think somewhere around 11.5" is about as short as you can get it and still have room for threading. My XCR is one of the older ones with a 1/8 barrel. We cut and rethreaded it @ 12.5 inches. I went this length because anything under that would not allow mounting of my sound suppressor, due to it contacting the gas valve. I also has to play with the gas system some. I ended up opening up my gas port and block a little to compensate for the decreased dwell-time of the gas in the short barrel. YMMV.
As for plans for XCR pistols, I don't know of any.