If it's anything like mine, it doesn't exactly unscrew; there should be a pin sticking out of the inner tube that sits in a dog-leg cut-out on the outer tube. You need to rotate the inner tube just enough to get the pin free of the dog-leg, and then it'll pull right out. The inner tube has to move in towards the receiver in order for that to happen because of spring tension holding the pin in the dog-leg. If everything is clean and slick, just turning the inner tube is enough to get it to move that way, but if things are a bit rusty and sticky, you might have to push the inner tube in hard before you'll be able rotate it.