First, every state is different. Some states are more strict against LE than the federal ruling.
By itself, an automobile is an exception to the warrant requirement. Depending on the reason for the search, locked containers can be forced open. Another factor is what is being searched for. You can't articulate forcing open a small safe when you are looking for a stolen propane tank (just a goofy example-the item/evidence being searched for has to be able to fit in the container)
An inventory is a blanket that allows LE to search everywhere provided they have a policy in place for inventory searches.
As for the firearms, it will just depend. If I pull you from the vehicle for a stop and have reason to believe a weapon is in the vehicle, I can search the immediate area, before letting you return, and secure the weapon.
We have been restricted by our state wanting us to obtain a warrant if the vehicle is parked, immobile, or unoccupied. We also cannot use the inventory search to get around the warrant requirement.
Each situation is different and laws vary so greatly, a blanket answer is not easy in this area. There are nazi states out there that have banned the SKS, so that alone could be an arrestable offense (although a Constitutional violation in my mind)