there are factors which prevent a low pressure propane (combustion starting from 1atm) from fitting into an airsoft replica. Pball guns can tolerate big expansion chambers which look goofy in an airsoft gun. You just can't scale them down unless you're willing to tackle some really difficult problems like developing an airtight seal which opens on detonation and can handle 1967C.
I actually did build some proof of concept airsoft propane combustion chambers a year or two ago. If you want to read up on a summary of work from that era of abandoned enterprise:
It's just not possible to get rapid flame propagation in a low pressure combustion setup that's slender enough to fit into an airsoft replica. Unless you're willing to make a Pancor Jackhammer which already looks like a pball gun.