Doc j: you are right. One possible solution is to activate the valve that lets the compressed air pass, with the compressed air generated by moving the piston to the right, in Jack's drawing. The piston does not need to hit the valve. I think a balanced valve is necessary so that it does not have opening force parallel to the axis of travel. The small spring closes it when once the piston has been pushed to the left, the pressure in the cylinder drops due to the air exhaust through the orifice.
It is necessary to solve what observed doc j, so that it works without problem.
Jack: The same as you think, I am thinking. With a little low pressure compressed air, and different diameters of the two pistons, you can get a lot of compression compression. I already plan to do some of this to test, but in the near future, when I finish improving the bb's loading system, it does work but it does have small drawbacks from time to time.