This would be more reliable.
This is just an idea, but I suppose it might work this way (95%) or with tiny modifications...
Only making a real design would give us a true answer! O-ring that acts like a valve is a floating O-ring. Because floating O-ring is not supposed to be used on rods, but only on pistons, I made it to be unstretched, except when in ready to shot state. That is achieved by use of the spring on the right side. In ready to shot state it is stretched 5% and it acts like a valve.

This is just an idea, but I suppose it might work this way (95%) or with tiny modifications...
Only making a real design would give us a true answer! O-ring that acts like a valve is a floating O-ring. Because floating O-ring is not supposed to be used on rods, but only on pistons, I made it to be unstretched, except when in ready to shot state. That is achieved by use of the spring on the right side. In ready to shot state it is stretched 5% and it acts like a valve.
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