If I'm not mistaken, in general:
Handgun - Locked and fully enclosed case, unloaded. So basically, to ready it, you would need to unlock case, take the gun out, insert a mag, and rack the slide.
Long gun - Must be unloaded. Does not need to be encased. I think a pump shotgun with a raptor grip or a pistol grip might be quick to load, just put a shell in the port and push the pump forward but still too long to handle well in the confines of a vehicle. The Stoeger Double Defense or other similar hammerless coach gun with the automatic safety disabled and the action already open might be faster yet.
Remember also, no mags over 10 rounds.