Missouri gun laws refer to both 'concealed' and 'concealable' firearms and things can get confusing because of this. Missouri also allows cities/towns to outlaw 'open carry.' This adds to the total confusion.
If you are at least 21, you can carry a loaded 'concealable' firearm (handgun) in your vehicle without a permit. If you are in an area that allows open carry (most of the state) you can keep this gun exposed or concealed. If you are in a city/town that prohibits open carry, you must keep the handgun concealed within a vehicle. If you aren't sure of the local ordinances, concealed is legal in all areas.
Long guns can be trickier. If you in an area of the state that allow open carry (again, most of the state), you may carry a loaded long gun in your vehicle without a permit as long as it is exposed. This means on the seat, in a rack, on the dash, etc. Very common in farm country. If you live in a city/town that prohibits open carry and don't have a permit, you may not carry a loaded long gun within reach or easily available. Probably best to have it unloaded too. We don't have a rule requiring casing a long gun, but it might not hurt in an anti-gun area. If you have a permit, loaded long guns can be carried in a vehicle concealed in any jurisdiction because the ccw portion of the law uses the term 'concealed' rather than 'concealable.'
You may now get your ccw at 21. It was 23 until a couple of years ago.
Confused yet? Except for a few cities that have open carry restrictions, Missouri is a great gun/hunting state to live in. I live in a rural area and always have a handgun and long gun in my truck. When the coyotes are active, my .223 in on the passenger seat, loaded, and ready to rock and roll.