Have you ever been hit by 2lbs of steel moving at 20+MPH? You may be in a otherwise minor accident that turned into a serious one because you did not secure that gun. Any heavy objects in your car or truck may become destructive to soft tissue and bone when in a car wreck.
In my truck, adding another 2-5 lb object to the mix isn't a big deal. With the shovel, axe, knives, hatchet, lug wrenches, chains, padlocks, hand tools, air compressor, frozen water bottles, and other skull-cracking objects in the cab.... the odds of a firearm causing damage are fairly low.
The risk is there. But, in my truck, I'd worry about some of those other things, first. (Like the axe, in particular.