Thanks this is an awesome guide to boost power for Umarex HDR50/TR50. One thing I would like to note is that most TR50 sold in USA is missing the flow regulator disk inside CO2 cartridge chamber. So one idea might be to remove the valve block (as per your instruction) and since valve block is cylinder of same diameter as the regulator disk, simply cut the valve block to appropriate length, drill a tiny hole and use it as regulator disk. On second thought, forget about making the regulator disk since it's not needed anyway to leap into HPA "multiverse". I also noticed your barrel extension. HDR50 being 50 caliber (actually slightly less than 0.5"), a schedule 40 3/8 inch steel pipe has perfect inner diameter for barrel extension (0.493"), and pressure rating. Just design a jig which clamps on the bottom rail and also screw in to slots at top and there is the perfect inexpensive barrel extension for HDR50.