There are so many things that make me want to say ''not a great idea''.
I'm a gun enthusiast, too, so don't take my comments to sound anti-gun.
Personally, I don't like the idea of a gun rolling around on a vehicle locked and loaded with the safety off.
It could discharge accidentally during a rollover or accidental contact with your culvert, shock from impact (a remote possibility, I acknowledge) could cause one.
The stability of the vehicle would be compromised, too.Â* A heavy item like that would affect the suspension quite a bit, the COG would be completely altered.Â* There are bound to be other issues I'm not thinking of at the moment.
Not to mention it just goes against every bit of firearms training I've ever had in my life, civilian or military, to let a hot weapon go out of my hands downrange, where I potentially can't control its discharge.