tl;dr, some quick math says doubling load time to one minute per round and requiring a bicycle pump seems fairly reasonable.
Also, we could add industrial-grade airguns that are way better, but uncraftable.
I can take a volume increase or leave it. I think it’s justified at the power levels we’re dealing with here, but it isn’t sufficient.
Some quick math:
Based on this gun: http://www.topairgun.com/50-air-venturi-wing-shot2-smooth-bore-air-shotgun-arrow
244cc air resivoir
3,000PSI
Based on a formula here: https://www.reddit.com/r/theydidthemath/comments/4qxdr8/request_how_many_pumps_would_it_take_to_fill_this/
(0.000244 * 19275000 * 75) / (287 x 303) = 0.054kg of air
"typical" bicycle pump produces 0.00035kg of air per pump.
0.054 / 0.00035 = 154 pumps per shot
Reference says 60 pumps / minute, or about 2.5min or 5x the current pump rate (also assuming a lot more leverage, more like a bike/truck ump instead of a builtin pump).
One one hand, this is a super-powerful gun, capable of downing a deer or maybe even boar in a single shot.
On the other hand, this isn’t representative of the cobbled-together hardware of the bolt driver, I find it very hard to believe that a survivor would be able to build a pressure resevior and valves capable of handling 3K PSI.
Drop that by a third to 1,000PSI, and it cuts load time to roughly one minute instead of 2.5, drops power output to match the existing bolt driver, and adresses my concerns about the survivor being able to build a gun that can handle that pressure.