In my story idea the main character is given a PCP airgun with a sort of self-charging mechanism in order to commit certain covert operations by assassinating certain individuals. (Don't ask, he was blackmailed into it.)
My question is, what would be a viable mechanism to refill a pre-charged pressurized airgun tank on the fly in combat without having to remove the tank and refill it using an air pump or refilling station?
Keep in mind the tech level of this world is only twenty years from now, so try not to come up with a solution that requires crazy material and tech advancements.
I would prefer the mechanism be as light and compact as possible and make very little noise. I would also like it not to require much effort from the user and the ability to be used when prone.
Note: A PCP airgun is an airgun that uses the pressure of a pre-filled pressurized air tank to propel a projectile. Most PCP tanks have enough air in them for twenty shots. However, I would err on the safe side and have it capable of refilling itself because he constantly on the move and wouldn't always have the time to find a compressed air filling station or break out a hand pump and start cranking. Also PCP tanks are somewhat large so carrying spares is hard. The reason he is using a PCP airgun is because of their low noise, heat signature, and recoil making them perfect for covert operations.
This picture is a picture of one of the more powerful PCP airguns on the market.
Just another side note for clarification: The airgun is designed to be semi-automatic for quick follow-up shots in case the character misses. I forgot to note the working gas being used is pressurized air so that the gun can be refilled using atmospheric air. The chamber pressure is in the range of 4500 to 5000 psi and the refill should preferably be done in under a minute.