It will be hard to build this pistol. You could only have a short stroke and a short barrel. The short stroke will take a bulky piston and a really stiff spring to get enough velocity. The stiffness of the spring will require a high force. The force will also make a simple trigger hard to pull (thatīs bad for the accuracy).The short barrel means a waste of pressure which also has to be compensated.
Your best bet might be a pistol with an air container thatīs pumped up to the required pressure.
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If it isnīt broken, it doesnīt have enough features yet.