I had open hart surgury, I built a 6mm Mongoose 18" prior to the surgury to produce a very low recoiling 100 grain bullet shooter. It is very close to a 6mm TCU. You trim 223 LC brass .050 and can load leavy bullets in an AR-15 mag at 3.000 fps. The gas operation mitigates the recoil. Easy to form and reload for. Mad Dog Wapins Systems (MDWS) sells the 1-8 twist SS barrels and dies. Mine shoots .3 inches with 80s. Good luck.
“To expect defeat is nine-tenths of defeat itself. It becomes a self-fulfilling prophecy. It is best to plan for all eventualities then believe in success, and only cross the failure bridge if you come to it." Francis Marion - The Swamp Fox
