Someone posted a few months ago asking about refilling them with air.
While CO2 has a lower pressure (about 56 Bar) and you can fill Air upto 300 Bar if you have a cylinder.
There are a couple of problems one is your valve and internals are designed to run at CO2 pressure.
So you would have to fill it to the same pressure as CO2.
This is completely pointless, because the CO2 is a liquid whereas the air is a gas.
For instance if you were to boil 18g of water (1 mole of water is 18g), then the gaseos water would occupy a volume 22.4 litres.
The density of water is 1 gram per ml.
So this means a gas occupies 1244 times more volume.
So if you are getting 50 shots from a CO2 capsule with liquid CO2 then you will be lucky if you get 1 shot with air at the same pressure.
So it is scientifically impossible to get this to work.
Sorry about that.
If you have a crosman co2 pistol or rifle like I do, then you can buy bulk fill CO2 kits from
http://www.crookedbarn.com/airgun_books1.htm
Then just buy a cheap large CO2 fire extinguisher from that internet auction site.
Then when eventually you run out of CO2, just get it refilled.
Obviously CO2 extinguishers are refillable.
I worked out the difference in price per shot a while ago when I was considering converting to bulk fill..
I buy 500 packs of CO2 from a very cheap source.
Even with very low CO2 Capsule price I worked out it was 14 times cheaper per shot with bulk fill.
If you don't know, bulk fill means filling the main tube with liquid CO2.