Baz, do you invert the extinguisher so you get liquid CO2 not just gas in the pistol, as the ones I have do not have siphon tubes. ?
I experimented with CO2 filling a while ago as I wanted to adapt an extinguisher to fill my CO2 Webley Senior. I bought a CO2 regulator and valve from an aquarium supplies company, and, with Laurie's help, have now got both Sodastream and Extinguisher filling adaptors.
I knew I had some old style full Sodastream bottles, but couldn't find them so got hold of a discarded new style Sodastream machine, and removed the valve assembly and lever which is perfect for filling pistols, as the inverted bottle just needs to be slightly pressed against a hard surface for the valve to operate.
I have since found the old style bottles, so can still use the adaptor Laurie made for me to do a similar job, but the bottle has to be screwed into the adaptor to release the CO2.