Here's the problem, Loosehead, technically, according to the Constitution, you could do as in your example. However, local laws (city, or even some State laws) sometimes prohibit your example. In most of our western States, we all carry a rifle on a gunrack in the rear window of our pick-up truck. If the State were signatory to anyone of the different kinds of "conceal/carry" (CCW) one could legally carry that gun on the streets openly. In most State's with CCW laws you are also permitted to carry a handgun in a concealed fashion, or openly, as well. A brief list of those States is � Alaska, Arizona, Idaho, Kentucky, Montana, New Mexico, Ohio, South Dakota, Vermont, Virginia, West Virginia, and Wyoming. As you can see, most are either western or highly rural in population demographics. Most other State's limit this right in different ways.
It really all comes down to location and intent. Confusing, I know, but it seems to work in most places. It is important to point out that noone can purchase any kind of firearm without a background check for every gun purchased. This isn't the same a registering the guns, which is strongly opposed, but the background check is quite thorough...