I don't know if Khanh fired his gun with shells, but mine is hard to open by just dry firing. Also these guns have a spring in the bushing, which when the gun is opened and starting to recock, the spring will push the firing pin back.
It is still quite possible that the firing pins are hanging up, but they are quite short and only protrude about 3/32".

If would be nice to know if the gun is hard to open dry fired.

David