There is no problem displaying guns on a wall if you have a secure location or they will bet be in plain view from the outside of the house. To do so otherwise would invite theft these days.
I display a few guns in my gun room on the wall and do so in the horizontal so the entire gun can be seen in profile. Others are stored in gun racks but they look like so many pens in a row. Hard to tell one from the other that way.
It gets down to how you want them displayed, in a case or on the wall in some type of custom rack. One thing to consider is lighting. It makes all the difference to the display in my book.
