Gun cases become more of a nuisance than a help when a necessary key is lost. If a stored firearm is needed, and access is denied, it can be quite frustrating. Some cases are weak enough so that a simple screwdriver can pop off the outside of the lock, but others are designed to defy even the most dedicated opening attempt. If that is the situation, the options are limited.
Look again for your lost gun-case key. Look in clothing normally worn during hunting or shooting firearms, even in boots. Looking in your firearm cleaning supplies and ammo cases. Ask all your hunting friends if they picked up your key by accident.
Take your gun case to a local locksmith, if it is transportable, or make an appointment for a locksmith to come to your location. If a locksmith knows the manufacturer of the case, he can usually open it and make a replacement key.
Look for a small number near the lock. If there is one, call the manufacturer with the number, and ask for a replacement key.
If you hide your gun case key, it is easy to forget where it is hidden. Tape a clue to the bottom of your case with an answer that only you will know. For example, "The cabinet where you found the dead mouse."