I am newly registered but far from a first-time-reader, I have been around as a quiet reader for about 2 years now

Finally I got around to building my own PCP, or at least attempt so. I have the basic components laid out but am struck at one problem:
I plan to use a Pressure washer handle as the main part for my first try.
First off; Is there any reason why there are so few of those used? I can remember JSR posting a pic of a small HPA bottle hooked up to one but thats about it. That makes me wonder because in my opinion they are quite ideal because of their insane max pressure ratings.
But on to my real question: The pressure washer I got in mind has an "M22" threading for connection to the chamber. Any Idea what sort of threading that is and if its possible to connect that to standard fittings?
Thanks in avance
SubZer0