As far as I know, and please check this with law enforcement before you do anything......
yes, you can have an OPEN CARRY
gun in your car in St. Louis if you are over 21...or it could be 23...check if these ages apply to your case. However, the gun cannot be concealed
and must be in plain sight at all times
Also, you should know how best to inform a police officer you have a gun in your vehicle should you get pulled over. If you don't inform the officer, you can be arrested, or worse, have your ass shot off. Contact your local police department and ask them how you should handle that type of situation.
I went through all this when I took my concealed carry in NC. There are usually some tricky laws and regs to carry in a car. Such as, what do you do with the gun when you’re not in the car? Can you put it under the seat? Is it then considered concealed even if you're not in the car? If you are in the habit of putting the gun under the seat or in the glove compartment when you are out of the car, I can guarantee you will forget it and drive around with it concealed and if you get stopped by a cop and he finds it....you're in the slammer.
Best bet is to find the actual laws for MO and the City of St. Louis and read them. It will save you a lot of hassle.
BTW, if anyone knows a site that has those laws posted, please post the URL...I'd really like to read the laws myself.