Practicality? - I don't see it either. But it would be cool, and there is at least one freak out there who would LOVE it
Maybe hinting at something almost resembling practicality - these little tanks would be easy to fill with one of these -> [LINK
We're not talking about refilling an expended 12g with HPA, we're talking about making a refillable HPA replacement for the 12g. Imagine a regulator from a HPA tank, attacked to a 1.25" OD bottle stuck to it.
Okay - so my math seems to have been off - I guess it would be a 4CI tank - however an 8CI tank would be very nice for testing relative efficiency of various guns.
I don't know if the $100 benchmark is the way to go... a 13/3000 tank is about $50, equating to 5,000 shots with an excellent gun for the same price. That $50 tank should result in about 150(+/-) in an excellent gun per refill. And I think (and may well be wrong) that the 13/3000 8/3000 and 4/3000 would be exempt from hydro. At any rate, most of the cost of the virtual 12g would be the reg.