[CobaltBlue]

>>what is the prevailing attitude in the MD area towards a citizen keeping a long gun such as an M1 Garand in his vehicle?<<

There are a lot of hunters in Maryland, but nobody drives around with a gun in a gun rack in the back of the pickup truck in the DC metro area. Too many thieves.

[Marauder]

"Too many thieves" is true even in Texas anymore; while I was growing up, the farm and ranch hands would park downtown with rifles and shotguns in full view in the back window of their pickups, doors unlocked, and no one had to worry about theft.
Now one must carry a rifle behind the seat back in a standard cab, or under the flip seat of an extended cab; and of course lock the truck. Out of sight, out of temptation.
I even have an alarm system that doesn't make noise; it sends a signal to my beeper, giving me a chance to catch the mutts and kick some butt before I call the law.