You can add a random number to the bullet's angle of motion.
It could be a simple random(-15,15) on every shot, this will make bullets to fly in 30 degrees cone.
Or you can do something more complex like this:
choose(0, random(-5,5), choose(-1,1)*random(20,30))
This will make some shots 100% accurate, some not too accurate and some very inaccurate.