The back of the XCR has what they call a receiver extension. In the pistol, it's just a rounded hunk of metal with a sling point (I think). To put a stock on, you need to have a receiver extension that either has the non-folding stock, or the half-hinge. I would recommend the half-hinge, unless you're in a commie state. The other half of the hinge would be either on the factory folding stock, or the M4 stock adapter. The adapter allows you to put any buffer tube and tube mounted stock on.
Although, with the SBR, you might want to just stick with the shortest LOP factory folder (9" maybe?).