I would not trust my life to frangible ammo. For all of the reasons stated above.
Look at it this way. If this is your car gun, why would you possibly want it to penetrate LESS than any other gun? If you have to use it, it means that you are in your car, you can't drive away from the threat, and now you have to shoot your way out. You may need to shoot through your door, THROUGH YOUR WINDSHIELD, or back into another car that is shooting at you. Not is not the time for minimal penetration, if there ever is one.