Bill 57
I have a 40 ftlb Air Ranger so the same probe as you. A couple of years ago Daystate sent me some freebie o rings. One of which was the pellet probe and it fitted very easily and must have been the 12 ftlb version, but still lasted two or more years so that's an option assuming you have an FAC Huntsman. Recently i needed to renew and ordered the FAC ones which are brittle. I took the entire cocking bolt out (not a big deal as had done it before), so gave me a lot more purchase and using a pin like a strong safety pin I got inside the seal with it as far up the probe as possible and just lifted it up slightly from one side used my fingers on the other side managed to roll it on.
It did seem at first I would never be able to do it, but its on and all is well. Good luck.

Golf224