Several different methods to do this but.....This is how I sealed the probe and TP.Sandwich an o-ring in the breech and push the barrel up against it.Will need to shave the breech end of the barrel about .050" so the TP ports align.This can be done with a file or on a lathe.Thread the plastic carrier piece 8-32 and make a new probe from brass.Taper it and cut it to length so it fits snug as a bug in a rug.For the TP made a 1/4" brass cap that fits over the nipple.It fits into the valve which is counterbored to fit.Used 3145 RTV and lace the assembly together.
Good call Mr. Ronno Can also drill it out and make a 22 cal 2100.
The early 766 receivers with the tapered shroud are the best.