CO2 is useful as an airgun gas because it can be compressed to liquid or solid at relatively low pressure (around 800psi at room temp). Conversely, components of air do not begin to liquify until well past 15kpsi. Compression to phase change (liquid or solid) provides very compact storage.
This means that you cannot acheive sane reservoir pressure (around 115psi for propane) AND useful compression ratio without CO2 or propane. GBBs tuned for CO2 are designed to run on regulated CO2 expanded from liquid. GBBs tuned for HFC134a or propane are designed to run from unregulated liquid-gas propellents.
Air is not a useful GBB propellant unless you're willing to carry a large HPA tank which stores gas at very high pressure and use regulation for your low pressure GBB. 12g caplets are not refillable as they have a puncture seal which is not reclosable.
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