Don't mind the thread nazi. ^^^ Some folks just really want to be moderators.
As far as I know, open carry of a handgun is legal in Missouri, with the exception of cities that have banned it (St. Louis being one). While I am not familiar with the statutes, even if legal, I assure you that long gun carry, especially in an urban area, will get you many strange looks, MWAG calls, and police attention. The fact that you are young and black will not help things. You will likely be detained. You may be arrested. You may have to pay lots of money to a lawyer.
None of the above makes long gun carry wrong. Just pointing things out as they are.
If you choose to seriously pursue this, I strongly urge you to comb through the Missouri statutes and St. Louis ordinances for anything that might be construed to prohibit long gun carry. I would also look at disorderly conduct laws. Actually, if it were me, I'd pay for an hour of an attorney's time and ask him about the ramifications.
Good luck.