I have an old Walther CM-2 CO2 powered match air pistol. I usually refill the removable cylinders from a 2Kg fire extinguisher, a 2Kg extinguisher costs £24 and last me a good while, about 12,000 pellets I guess, so quite cheap to run. I have just had the cylinder refilled locally while I waited at a local pub gas supplier for £5. I reckon that's a bargain.
That is a good price, I normally buy the complete full 2kg extinguishers for £10 each. There are a lot around as they have a sell by date for use in fires even though there are as new.
Baz
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