What gun ? Assault rifle calibers or handgun calibers ? Handgun calibers from a handgun or from an SMG ?
I don't have any experience with your bitumen. Admittedly I'm skeptical. I guess maybe it will stop a little 22 rimfire (I haven't tried)..... but 5mm of bitumen vs a high powered rifle round ?? As far as I know, from my time with gun sports back in Australia, nothing much in a standard car will stand up to high powered rifle fire,....except the engine block. We shot through everything with .222 and .223 (5.56mm). And that's referring to the sheet metal on the car doors. Lengthwise, a 5.56mm will easily shoot through your boot, right through your seat and through you all the way to the dashboard...although in many cases the 5.56mm bullet's trajectory is deflected as it penetrates because it's a light fast stepping round. Of course if you go up to 7.62mm NATO, that will just cut through everything like hot knife through warm butter.
Also, for obvious reasons, you can't bitumen windscreens, and I have not seen bulletproof windscreens that will withstand repeated hits from an AK47 at close range....but then again, there were not meant to anyway. Just enough time to drive away.
