because the outside of the .22 barrel liner can't be larger in OD than about .300". The bore diameter of 308 is about .302", and you need at least .002" to keep the liner from getting stuck in the bore. Cut a .223" bore thru a .300" od rod, and you have .077", and the wall thickness of the liner is only half that thick. So you end up .038" walls, you can bend that liner like a pretzel in you bare hands. If you don't CAREFULLY install the chamber-insert in the chamber (relative to the numbers on a clock) you will get WILD variations in POI on the target.