I like the Geissele, but I'm thinking adjustable. I have the tech to pin any block if I need to. I mocked up the 12-1/16 tube in the upper assembly, 7/8" long, so need a 11-3/16 tube to hit dead center of the receiver cutout. Other option is to move the hole back, the gas block would block the original hole while it gummed up with carbon, or I could drill/ tap a set screw into it. Thinking the custom tube is the easiest option.