Thanks to IJ and some assistance from PhatMan (to correct IJ's smelin mistook) Binners has been able to contact me by email and I have replied - hopefully with some information that could help him find a suitable Walther CO2 cylinder for his magnificent match rifle.
I always did like the Walther GG90 CO2 match rifles - but I know that the longer length CO2 cylinders are a "bit thin on the ground" and are not so easy to find as the Walther CO2 pistol cylinders.
The scarcity of these "rifle length" CO2 cylinders is quite well known, and I have been offered a CG90 in the past with a pistol cylinder fitted as these are a lot easier to find and will still hold enough CO2 to give quite a few shots. This is not quite as daft as it may at first appear because CO2 runs at a lower pressure than air and gives lots more shots for a similar volume - so it would actually work reasonably well - but the steel CO2 cylinders are quite heavy and so a shorter pistol length substitute would noticeably change the balance of the rifle - possibly for the better!
Please don't ask IJ about his CO2 cylinder charging technique/experience/success unless you have a lot of spare time and no other ambition for the next few years of your life.
Both the Walther CO2 rifle and pistol cylinder share the same fitting, but a suitable socket (fitted to a long shaft) would be needed to tighten or remove a pistol cylinder if it was fitted inside the rifle cylinder tunnel as it would be very difficult without a tool like this. Welding a socket onto a steel shaft would make a suitable tool.
Hope this helps too!
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