A much easier way to relieve mainsprings on a A&D boxlock is to:
- break the gun open
- pull the triggers
- close the gun while pulling on the triggers (without shells in the chambers of course)
- take the gun apart if needs be.
That's it. No snap caps, no block, no tools required.

It is not guaranteed to work on sidelocks or other actions.

I know people who do this every time they store their gun, but I must say that I don't think that it is any useful.

Best regards,
WC-