Supernatural's 1967 Chevy Impala is the only home Dean and Sam Winchester have ever really known, so it makes sense the boys have every weapon they might ever need in the trunk. As the Winchester brothers traverse the U.S. hunting monsters, the Impala is their constant companion. Since a surprising amount of equipment is needed to «gank» baddies, they've also turned it into a mobile arsenal. Whether it's the famed Colt or a run-of-the-mill shotgun, the trunk is always packed to the brim with whatever they might need.
The Impala was first introduced in the pilot and only a few minutes in, Dean pops the trunk to dig out some monster research. The trunk is revealed to have a false bottom where weapons are stored, presumably to prevent the Winchester brothers from immediately getting arrested if pulled over. It's not a foolproof system, but it is lifesaving. The brothers are often in need of a specialized weapon to kill whatever monster they're facing so when an unexpected threat appears, it's usually a short run to the car to fetch whatever they need.
Related: Supernatural's Series Finale Ending & The Winchesters' Fates Explained
A catalog of every weapon in the Impala reveals an improbable number of items to be constantly stored. With a limited amount of storage space, the Winchesters presumably acquire and retire weapons as needed, throwing out broken blades or guns in favor of new equipment. Some more advanced weapons make basic tools redundant, while other weapons are only stored for a short time. In Supernatural's 15 seasons, the Impala's trunk was used to store survival supplies, basic hunting tools, knives and blades, a surplus of firearms, and some rare specialty weapons. It's every survivalist's dream.
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