When do we get an XCR pistol? ??? ??? ??? I've been building a couple of AR pistols, but it sure would be nice to have an XCR pistol!
How short can you reliably make the barrel?
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.