SpudFiles

As with every piston design, an adequate buffer is required. There are quite a few things you could try using to cushion the piston, just try looking in a lot of the piston gun threads. Much of the stuff you will need for this thing has already been developed, mainly the little details to make this baby sing. Now, I haven't made my own piston yet, so I don't remember any materials off the top of my head.

As I see it, the problem is one of surface area.

Your loading piston has too much area for the pressure to act on. Thus it is being slammed open rather forcibly.

If you can reduce the diameter of the bolt by half, you will decrease the surface area by a factor of four.

You will be able to use a lighter spring, but you will still need a bumper on both ends to cushion impacts.