Suppose one of your design goals was high efficiency for lots of shots from each fill. You must locate the tank behind and coaxial with the barrel for maximum efficiency. So the tank will double as a stock in the most efficient design. A larger tank can also be used in this position compared to mounting it out front, as most PCP air rifles do. Assume another design goal is simplicity and a low parts count.
Conventional guns use two valves.
Two o-rings, both the same size, seal the valve cover. No other o-rings are used. There is no need for a conventional bolt. The hammer spring is large, and lightly stressed, with 4 large Delrin spacers, leaving a lot of room for adjustment of spring preload and hammer weight. When I say easy to make, they would be easy for you to make them by hand yourself if you ever had to.