I recently sold the upper off my 20" Bushmaster with the fixed butt stock and am planning on buying a dissipator upper for it. Along with that, my plan is to put a collapsible stock on it, perhaps something from Magpul, like their CTR. What all do I need to replace going to a collapsible stock? The tube, spring and buffer? Are any of the parts re-usable? I've looked around but can't seem to find any "kits". If you have any recommendations on stocks I'd appreciate them. Thanks in advance!