Hinz, please correct whatever I may get wrong. The custom air tube is scratch built from stainless steel tubing that is 0.157” ID x 0.171" OD (stock 1938B is 0.078" ID x 0.163” OD). The custom air tube requires the '38B abutment seal be carefully reamed oversize to give a good snug fit to the air tube OD. This custom tube is what Hinz has used to get the highest recorded MV from a repeater to date. The Daisy factory air tube can be bored out to a max of 0.125" as long as it's a new style air tube made from 'seamless' tubing. The older tubes had visible seams and these tubes are not suited to boring o/s. You might get away with 3/32" but much beyond that you'll often run into trouble.