I just learned about these and they seem to be a viable option for HD. I have a few questions, especially concerning the buttstock or buffer tube. What;s the purpose of this tube? Does it aid or help stabilize the pistol when shooting it?? Is it adjustable in length or do they sell ones that are? What is the max length these tubes come in? Can this tube serve as a mock stock in that you can press it to your body, as long as you don't modify it in any way?