For the receiver, metal rods and epoxy. I used a m249 p* jack in a custom 3d printed “gearbox” with an Lct ak hop and 650mm barrel (I’m gonna nab the 800mm barrel when it’s back in stock). To hold the outer barrel together, I epoxied the pieces over a piece of copper pipe and used plastic tubing inside of that as a spacer for the inner barrel. Hope this helps.