Does the law differ if you put your guns in the uncovered bed of your truck?
No, I routinely haul numerous rifles in the back of my open Jeep. It's not a good idea to park it unattended somewhere, but there's no law against it because technically the weapons are still in the vehicle. There's nothing in the law that says the vehicle must have doors, windows and/or a top!
Speaking of which, I also go to the local military base where I shoot with the gun bags openly visible when I go through the gate. I have yet to have the guards say anything other than "have fun shooting!" It is the only base in the area that actually has read and understands 18 U.S.C. §930!
Cheers! M2