The idea itself is good, BUT THE ONLY GUNS ALLOWED TO BE SHOT ON CARS WOULD BE PISTOLS AND SUBMACHINEGUNS. This way, we would have:

P2K, PDW SD, MPX SMG, M1911, USP .45 TAC, MP5A5, SKORPIAN EVO, SOUIZEN, UMP 45, SPECTRE, VECTOR, AF 2011, M3 GREASE GUN, ION SMG, ION PISTOLS, PHOTON SMG, FLUX SMG, ANTIMATTER SMG, PLASMA PISTOLS, P90, FNP90, STEN, TEC 9, MAGNUM .44, DESERT EAGLES, AP7, KAP 40, MK23, SOCOM MK23, GLOCK 18, MASCHINENPISTOLEN 40, FOG, FMG9, AUTO PISTOLS, AK74U SD (yes this is a smg), FIVE SEVEN, K180 and every other smg / pistol i'm missing