Great Ernest, thanks again. I removed my OEM thumb screw and replaced it with an Allen headed grub screw which I torque down each time with my Fat Wrench inch/pound torque driver to achieve exactly the same tension on the barrel each time to avoid the possibility of the barrel moving to the left and contacting the pellet probe. Also as you point out the thumb screw only needs enough tension to prevent the barrel moving forward as the locating notch at the end of the barrel prevents the barrel rotating and misaligning the transfer port. The problem is applying the same pressure each time the barrel is fitted, to maintain consistency in POI and probe to barrel clearance.
So far I haven't had the grub screw come lose, but I may be over tightening it and causing slight barrel/probe misalignment.
I like your idea of the outer delrin washer to contain the Oring preventing the Oring over expanding outwards when the thumb screw is tightened down reducing the Orings effectiveness in maintaining resistance the prevent the thumbscrew loosening. Also the Delrin washer's thickness can be varied so that it limits how far the thumb screw can be tightened down on the barrel, also preventing barrel/probe misalignment and possible POI change from over tightening, yet the Oring preventing loosening.
Thanks again Ernest, it's a great idea.
Ross