Road Gunner Achievement

This should come naturally as you play through the game. You'll get quite a few right at the start of game during the "No Way Out" mission. After that, you'll have plenty of opportunities as the cult are driving all over Hope County and you'll most likely be in a car driving or a passenger.

To shoot while driving, simply press to pull out your gun and shoot. If you're a passenger or auto driving then press / to lean out of the window and to shoot.