On my original MG47 prototype I made a gas tube out of a section of a 12ga. barrel and a piston to fit. I did have to slightly open up the RSB to get the larger piston to pass through. Then with a very tight gas tube to gas block joint, I had just a little too much force. The BC would slam back into the butt stock, which was also the BC stop, with enough force that it started to egg out the butt stock attachment pin holes. I then drill some gas relief holes in the gas tube but it still gave me more that enough force to actuate the feed get full stroke from the BC and operate the feed cam. An adjustable gas block would be a nice feature.