Transporting a firearm is regulated, and that framework is not just a question of car boot space. The principles found in French law fit into a few points: a legitimate reason for the trip, an unloaded firearm, transport in a case or packaging, and ammunition kept separate. If you are outside France, check your local rules: the substance is similar, the details differ.
The legitimate reason covers the expected situations: travelling to the range or to a hunting ground, visiting a gun shop, moving a firearm during a house move. A trip with no link to the sport is precisely what the framework is meant to prevent.
This guide sets out general principles and good transport practice. It replaces neither the official texts, nor your federation's guidance, nor your local authority's information, which alone are authoritative for your situation.
A hard case protects against knocks and crushing: the logical choice for a shotgun or rifle that travels often, especially if other objects share the boot. Its shell holds the shape and stops the gun from moving.
A soft case is lighter and easier to store, but it does not absorb impacts and passes vibrations through. It suits a short trip, with the gun alone in the boot, provided the inner foam is clean and dry.
A hard box with a lock or combination adds a closure and a handle: handy for a handgun, safer against accidental opening. Check that it is big enough for the gun without compressing it, and that the foam is not soaked.