1/ unscrew the end of the barrel (tx200hc) and poke out the remaining broken rubber stop, it just drops out the barrel end.
2/ I trimmed off the thin rubber tail leaving the thick stub still on and dropped the rubber buffer in boiling water, (watch those fingers)

Leave in there to soften for a couple of minutes
3/ then i just screwed it in with my thumb

Ps: if you can't get it to fit in, Plan B

Get some hozelock o rings ebay item number 200345021257
And pop one or two onto the cocking lever
Hope this is of some use to someone

Mic