If you could build one of these to take out tailgaters, I'll take two, but it needs to be mounted low with shielding for the tank when it launches.. You could tie it to a short, wide monitor across the bottom of the rear glass that reads "If you can read this, you're too close" with a countdown from 9 underneath..
I had an idea about that also.
I read somewhere about it being possible to adapt a Remington 1100 to be belt fed. There wasn't enough detail for me to know if it would work, but it seemed unlikely to me.
However that didn't stop me from thinking the hot setup would be to have three mounted low in your trunk and firing out somewhere near bumper level, angled up just a bit.
A 3 round burst should be enough to discourage most tailgaters.