Firearm is visible from more than one angle. Unfortunately that "angle" is not defined. I would cite it but I forget where it was that I read it, should have cataloged it.
Now why is it defined in such a way? Visible in your vehicle is open carry as well and you can get charged for the rifle in the gun rack of the pick up.
Now lets take a close look at the question, you have asked for legal advice, no one can give you that but a lawyer.
My own interpretation of the laws I have read on open carry are a mix of open and concealed carry statutes. In other words, if it does not qualify as concealed carry or container carry, it is indeed open carry IMHO, you should seek a lawyers professional opinion for your own decisions.
1. Concealed means covered up so that no part of the firearm is visible, not to be mistaken with printing laws of other states IE if it is under your shirt but anyone can tell it is a firearm it remains a concealed firearm, other states vary.
2. If it is in a container IE gun case or other package that completely encloses the firearm where no part is visible it is container carry, though I do not recall ever seeing statutes about container carry for the state of Missouri, more the federal transportation law.
3. If it can be seen, any part of the firearm, it is open carry.
Again, this was not legal advice, only your own lawyer can give you that, and yes, you should ask one, your freedom could depend upon it.