MD statute 4-203 gives the relatively small list of exceptions to the carrying of weapons in your car. They include:
Going to and from a gun shop
Going to or from a hunt
Going to or from an organized shoot or range
read it here
Driving from one house you own to another house you own is also ok. But if you are driving from VA to a home in MD you don't own, technically you are in violation I would either travel in compliance with FOPA (locked in trunk or in car with locked container unloaded) or tell the LEO you are coming back or going to an organized shoot or to the range. You are then legal.
If you are traveling in MD all you need to be legal is to have the firearm unloaded in an enclosed holster or case and be involved in one of the activities in listed in 4-203.
Contrary to some of the incorrect info given here, loaded mags are completely legal. Two places to avoid when getting legal info, cops and gun shops.