answer will be soundproofing....
traditional sound proofing material used are bitument. If you have friends in construction line, then you can get some sample of those bitument sheet.... they usually use it on the roof.
Those bitument sheet are measured in mm, im personally using those alluminium backing bitument sheet which is 1.5mm. Total 3 layers, that would make it 4.5mm....
bitument are soft, however, it is sticky and elastic, when the thickness is thick enough, the bullet will not be able to shoot thru the bitument....
You can try to paste a few layer or bitument (total around 5mm) on a piece of metal sheet (same thickness as your car body panel), ask your friend to shoot on the metal sheet and you can see what i meant....