The first thing to consider if using someones vehicle is muzzle blast. It will screw up a paint job bigtime. The second is appendages. Radio antennas (if shooting across the hood) are often not seen in sights/scopes because of their proximity and rear view mirrors (when shooting out the window) are often lower than line of sight of the sight but not bullet trajectory at muzzle. In a life or death situation bullet deflection is an issue. If it's just a practice session with your every day ride it will cause problems like being pulled over because your rear view mirror is swinging off the side of your vehicle. Addressing the paint/antenna/mirror thing gets a little spendy also which cuts into the old shooting budget. Did I mention the ribbing you're going to take from friends/family/SPOUSE. Beginning to sound like a first hand account? Must be your imagination.