Like Madmax said the combustion chamber takes space, but is that really an issue?
Many guns have a full stock version, which is known and if you're willing to have an external system (Just toss it in a bag on your back with a hose), then couldn't the combustion chamber be lodged in the stock?
When we're using this kind of system, we really have no reason for a big battery, although a small one for loading the BBs and sparking the gas would be an idea.
Could this not be possible? Madmax?
RS Type 56-1 & RS Type 97B & KWA M11A1 w/steel kit
KJ M1911A1 & KSC G19 + AI Grenade