You can google this info online..the NRA has a site that answers this from State to State, as laws may vary from State to State.
Here in Maryland, PA, and West Va, you can transport your long gun in a truck rack and not cased, as long as it is not loaded and you are going Hunting or to a sporting event or such.
I prefer to carry my rifle or shotgun in the rack so It doesn't take up room on the floor of my SUV or in the back area. Also it keeps the scope from banging around with the case sliding around. I never put it in the rack and leave it unattended for thieves, so it is not a concern for carring it out in the open..having tented windows helps also. Keeping your Ammo in a lock box also makes it difficuilt for fast loading and this is what law enforcement is mostly concerned about. Heres a picture of my gun rack that I built for my SUV, it will carry 4 long guns very easily..2 on each side..very convient for traveling with friends and saving cargo room.